Short answer
- The best current evidence says the observable universe holds a few hundred billion galaxies, per a 2024 analysis of the universe's background glow by Marc Postman and colleagues. A 2016 study led by Christopher Conselice estimated at least 2 trillion, but later measurements found no sign of that many hidden galaxies. The figure is an estimate, still being refined.
Why this question is harder than it sounds
“How many galaxies are there?” sounds like a question with one number for an answer. In practice it has had several answers, and each one came with a date and a method attached. Astronomers have never counted every galaxy one by one. They count the galaxies they can see in small patches of sky, then use careful math to estimate what the whole sky must hold. Every published figure is therefore an estimate, and the honest version of the answer always names who made it, when, and how.
This matters because the numbers you see quoted online come from different eras of astronomy. A figure from a 1996 textbook, a 2016 press release, and a 2024 research paper can all be “correct” in the sense that each followed from the best data available at the time, yet they disagree by a factor of ten. This article walks through the whole story: where each estimate came from, how the counting actually works, what the different types of galaxies have to do with the total, and what remains unknown.
A short history of the count
1995: Hubble stares at “nothing” for ten days
The modern counting effort began with a gamble. In December 1995, the Hubble Space Telescope was pointed at a small, deliberately unremarkable patch of sky in the constellation Ursa Major for ten consecutive days, from December 18 to December 28. The field had been chosen to be as empty as possible so that Hubble would look far past the stars of our own Milky Way and the nearby galaxies in front of it.
The result, released on January 15, 1996, was astonishing. The Hubble Deep Field showed almost 3,000 galaxies in a region of sky only about 2.6 arcminutes across, built from 342 separate exposures with a total exposure time of more than 100 hours. The distant galaxies looked smaller and more irregular than the galaxies near us, which astronomers read as evidence that galaxies grow by the gravitational merging of smaller pieces. This one image gave astronomers their first real grip on the universe’s galaxy population, and extrapolating from it produced the textbook figure that stood for two decades: roughly 200 billion galaxies in the observable universe.
2004: The Ultra Deep Field goes deeper
In 2004, Hubble returned to the deep-field idea with better cameras. The Hubble Ultra Deep Field, taken in the constellation Fornax with the Advanced Camera for Surveys, revealed about 10,000 galaxies in an area of sky roughly one tenth the diameter of the full moon. It remains the deepest portrait of the visible universe ever achieved, showing some galaxies as they were just half a billion years after the Big Bang. Because light from the most distant objects is stretched so far by the expansion of the universe that it drops out of visible light altogether, this image also marked a limit: to see further, astronomers would have to look in infrared.
2012: The eXtreme Deep Field
Published in 2012, the Hubble eXtreme Deep Field was not a new set of observations but a combination of more than 2,000 existing exposures of the same small spot of sky, totaling 22 days of exposure time (50 days of observing time). It pushed the visible-light limit as far as it could go before the infrared era took over.
2016: The two trillion estimate
In October 2016, a team led by Christopher Conselice of the University of Nottingham published a study in The Astrophysical Journal that revised the count dramatically upward. The team converted Hubble deep-field images into three-dimensional maps so they could measure the number of galaxies at different epochs in the universe’s history, then used mathematical models to infer the existence of galaxies too faint for Hubble to see directly.
Their conclusion: the observable universe contains at least 2 trillion galaxies, about ten times more than previously thought. According to the study, the missing 90 percent of galaxies are too faint and too far away for current telescopes to detect. The team argued that the early universe held far more small, faint galaxies than it does today, and that most of them merged over cosmic time into the larger galaxies we see now. In a NASA news release, Conselice said the results were powerful evidence that significant galaxy evolution had taken place, dramatically reducing the number of galaxies through mergers. The same release noted that the finding helped explain an ancient astronomical puzzle: why the sky is dark at night.
2021 and 2024: The background glow pushes back
The two trillion figure came with a testable prediction: if trillions of galaxies exist, their combined faint light should fill the universe with a faint glow called the cosmic optical background. Measuring that glow from near Earth is nearly impossible because sunlight reflecting off dust in the inner solar system drowns it out. So a team led by Marc Postman of the Space Telescope Science Institute and Tod Lauer of NOIRLab used NASA’s New Horizons spacecraft, more than 4 billion miles from Earth, where the sky is ten times darker than the darkest sky Hubble can access.
Their results, presented at a meeting of the American Astronomical Society in January 2021, set an upper limit on faint, unresolved galaxies: they number in the hundreds of billions, not 2 trillion as the 2016 study had suggested. As Lauer put it, take all the galaxies Hubble can see, double that number, and that is what the measurements support, but nothing more. Postman added simply: “We simply don’t see the light from two trillion galaxies.”
A refined follow-up, published in The Astrophysical Journal on August 28, 2024, went further. After correcting for all known light sources, including dust-scattered Milky Way starlight calibrated with data from the European Space Agency’s Planck mission, the researchers found that the remaining visible light was entirely consistent with the light generated by all galaxies over the past 12.6 billion years. They reported no evidence for significant levels of light from sources unknown to astronomers. In short, the glow of the universe is fully accounted for by the galaxies we already know about, which leaves little room for a hidden population of trillions.
How astronomers actually count galaxies
The method is straightforward in outline and painstaking in practice. Astronomers count every galaxy visible in a deep, well-studied patch of sky, then multiply that density by the total area of the sky. The Conselice team added two refinements: converting the two-dimensional images into three-dimensional maps so they could count galaxies at different distances and cosmic epochs, and using models of how galaxy populations change over time to infer how many faint galaxies must exist beyond what telescopes can resolve.
The strength of the method is also its weakness. Counting what you can see and multiplying is only as good as your corrections for what you cannot see. Galaxies can look faint because they are genuinely dim or because they are extremely distant, and the deep-field method is biased toward whatever a telescope can detect. That is why the 2016 study’s “at least 2 trillion” was explicitly a lower bound built on models, and why the New Horizons measurements matter: they measure the total light directly instead of modeling it, giving an independent check on how many unseen galaxies can plausibly exist.
There is a second reason the count moves around: astronomers are not all counting the same thing. Some estimates count only galaxies bright enough to be detected in a given survey. Others, like the 2016 study, count every galaxy the models say should exist down to the smallest dwarf galaxies. And the boundary of what counts as a separate galaxy can blur when small galaxies are mid-merger. Any figure without a date, an authority, and a method is, strictly speaking, incomplete.
The observable universe versus the whole universe
Every figure in this article refers to the observable universe, the part of the cosmos from which light has had time to reach us since the universe began. That is the only part we can measure. What lies beyond it is, by definition, unobservable to us right now.
This creates a subtle point that confuses many readers. The observable universe is roughly 93 billion light-years across, even though the universe itself is only about 13.8 billion years old. That is not a contradiction: space has been expanding the entire time light was traveling, stretching the distances between us and the most distant sources. NASA notes that the universe formed around 13.8 billion years ago, which is the age that sets the limit on how far back we can see.
As for the whole universe, beyond the observable part, we do not know its size. It may be finite but far larger than what we can see, or it may be infinite. If it is infinite, then the number of galaxies in it would be infinite too. That is why careful astronomers always attach the word “observable” to their counts, and why this article does the same.
The types of galaxies being counted
Not all galaxies are equal, and that unevenness is a big part of the counting story. NASA describes galaxies as collections of stars, planets, and vast clouds of gas and dust, all bound together by gravity. The largest contain trillions of stars and can stretch more than a million light-years across, while the smallest hold only a few thousand stars and span just a few hundred light-years. Most galaxies are between 10 billion and 13.6 billion years old, with some nearly as old as the 13.8-billion-year-old universe itself.
Astronomers sort galaxies into three main families by shape. Spiral galaxies, like our own Milky Way, have arms that curl around a central core and are often bright with young stars. Elliptical galaxies are smooth, rounded swarms of mostly older stars, and they include the universe’s largest galaxies. Irregular galaxies have no defined shape or structure and are typically smaller, though they can be very bright with active star formation.
Then there are the dwarf galaxies, the small fry that drive most of the disagreement in the counts. NASA’s overview notes that the Milky Way’s neighborhood includes dwarf galaxies with up to a few billion stars each. Individually faint and easy to miss, dwarf galaxies are thought to be enormously numerous, and they were the main ingredient in the 2016 study’s jump to 2 trillion. The problem for that estimate is that if as many dwarfs exist as the models predicted, their combined light should show up in the cosmic optical background, and the New Horizons measurements say it does not, at least not at the predicted level.
Most large galaxies, NASA adds, have supermassive black holes at their centers, some with billions of times the Sun’s mass. These giants anchor the bright, easily counted end of the population, the part of the census nobody disputes.
Our own place in the count
Zooming in helps the scale feel real. Our home galaxy, the Milky Way, is a spiral with a disk of stars spanning more than 100,000 light-years. Our solar system sits about halfway from the galactic center along one of the spiral arms, and it takes about 240 million years to complete a single orbit around the galaxy.
The Milky Way is not alone. It sits in a neighborhood of more than 50 other galaxies called the Local Group, ranging from small dwarf galaxies up to Andromeda, our nearest large galactic neighbor. The Local Group in turn sits near the edge of the much larger Virgo cluster and is part of the Laniakea supercluster. This nesting, groups within clusters within superclusters, with vast empty voids between them, is what astronomers call the cosmic web, and it means galaxies are not spread evenly through space. Any counting method has to account for that lumpiness, because a deep field that happens to look through a dense cluster will count more galaxies than one aimed at a void.
Why the night sky is dark
If the universe holds hundreds of billions of galaxies, each with billions or trillions of stars, why is the night sky dark? The NASA news release for the 2016 study called this “an ancient astronomical paradox,” and the answer has several parts.
First, the universe is young relative to its size. Light from the most distant galaxies is still traveling toward us, and much of it will never arrive within a human lifetime. Second, the universe is expanding, which stretches the light from distant galaxies toward longer, redder wavelengths. As the European Space Agency explains, for very distant objects the light is shifted so far that it drops out of the visible spectrum altogether and can only be seen in infrared. That light is real and measurable by instruments, but no human eye will ever see it. Third, galaxies are small and far apart on the cosmic scale. Even hundreds of billions of them cover only a tiny fraction of the sky as seen from Earth.
So the darkness of the night sky is not evidence against a crowded universe. It is what a young, expanding universe full of distant galaxies looks like.
What we still do not know
For all the progress, the count is unfinished business. The biggest open question is the faint end of the galaxy population: exactly how many small, dim galaxies exist, and how their numbers have changed over cosmic time. The 2016 study and the New Horizons measurements disagree on this, and the disagreement is exactly where new data will matter most.
The James Webb Space Telescope is now operating in the infrared wavelengths where the most distant, most redshifted galaxies appear, and astronomers are using it to probe the faint populations that Hubble could not resolve. As the 2024 background-light analysis noted, astronomers in the Webb era measure the cosmic optical background to look for light from sources other than known galaxies, and so far they keep finding that known galaxies account for it.
Beyond that, there are questions no telescope can answer yet. We do not know whether the universe as a whole is finite or infinite, which decides whether the total number of galaxies is merely enormous or literally uncountable. We do not know exactly how the first galaxies assembled in the first billion years after the Big Bang, though deep-field images keep pushing that frontier back. And every estimate remains hostage to the same fundamental limit: we can only count what light reaches us, corrected by models of what it misses.
The responsible way to read any headline number, then, is as a snapshot of a moving target. Around 200 billion was the best answer in the deep-field era. At least 2 trillion was the best answer in 2016, on the assumption that faint galaxies filled the gaps. A few hundred billion is the best answer as of 2024, now that the universe’s total glow has been measured directly. The next decade of infrared observations will move the figure again, and that is how science is supposed to work.
Frequently asked questions
How many galaxies are there in the observable universe?
As of 2024, the best-supported estimate is a few hundred billion. A 2016 study by Christopher Conselice’s team estimated at least 2 trillion, but follow-up measurements of the cosmic optical background by Marc Postman, Tod Lauer, and colleagues found that the universe’s total faint glow is fully explained by known galaxies, leaving little room for trillions of hidden ones.
Why did the estimate jump from 200 billion to 2 trillion?
The 200 billion figure came from counting galaxies visible in Hubble deep fields and extrapolating. The 2016 Conselice study added a model-based correction: it estimated how many small, faint galaxies must exist beyond Hubble’s detection limit, especially in the early universe, and concluded that the true total was at least ten times higher. Later direct measurements of the universe’s background light suggested that correction was too large.
Can telescopes see all the galaxies that exist?
No. Even the deepest Hubble images capture only a fraction of the galaxies in their patch of sky, because the rest are too faint, too distant, or too redshifted into infrared to register. The 2016 study estimated that 90 percent of galaxies were beyond Hubble’s reach, which is why astronomers rely on statistical models and independent checks like the cosmic optical background.
What is the difference between the observable universe and the whole universe?
The observable universe is the region from which light has had time to reach us since the universe began, about 13.8 billion years ago. The whole universe extends beyond that boundary, but we cannot observe it, so we do not know its size. It could be finite and much larger than the observable part, or it could be infinite, in which case the total number of galaxies would be infinite too.
How many galaxies are near our own?
The Milky Way belongs to the Local Group, a neighborhood of more than 50 galaxies according to NASA, ranging from small dwarf galaxies to Andromeda, our nearest large galactic neighbor. The Local Group sits near the edge of the Virgo cluster and is part of the Laniakea supercluster.
Will astronomers ever know the exact number?
Probably not exactly, because the count depends on definitions (what counts as a separate galaxy at the faint end) and on models of what telescopes cannot see. But the range is narrowing: each generation of instruments, from Hubble’s deep fields to New Horizons’ background measurements to the James Webb Space Telescope’s infrared surveys, tightens the uncertainty.
Sources & references
- NASA Science, "Hubble Deep Field Image at Full Resolution" (observation dates December 18-28, 1995; release January 15, 1996; field size 2.6 arcminutes; constellation Ursa Major).
- ESA/Hubble, "The Hubble Deep Fields" (1995 Christmas observations, 342 exposures, more than 100 hours, almost 3,000 galaxies; Hubble Ultra Deep Field 2004; eXtreme Deep Field 2012; redshift explanation).
- Space Telescope Science Institute for NASA, news release 2016-39, "Hubble Reveals Observable Universe Contains 10 Times More Galaxies Than Previously Thought" (October 13, 2016; Conselice team; at least 10 times more; missing 90 percent too faint; 3-D conversion method; Ultra Deep Field 10,000 galaxies in area one tenth the Moon's diameter).
- NASA Science, "Galaxies" (definitions, sizes, spiral/elliptical/irregular types, ages 10-13.6 billion years, universe age 13.8 billion years, supermassive black holes, Local Group of over 50 galaxies, Milky Way disk over 100,000 light-years, 240-million-year solar orbit, cosmic web; page updated August 12, 2026).
- Phys.org (NASA/STScI/JHUAPL release), "New Horizons spacecraft answers the question: How dark is space?" (January 12, 2021; Postman and Lauer; cosmic optical background; hundreds of billions, not 2 trillion; AAS presentation January 13, 2021; New Horizons 4 billion miles out, sky ten times darker than Hubble's).
- EarthSky (edited from NASA-published release), "New Horizons measures the darkness of deep space" (August 30, 2024; Postman et al., The Astrophysical Journal, published August 28, 2024; background light consistent with all galaxies over the past 12.6 billion years; no evidence of unknown light sources).
- Wikipedia, "Observable universe" (diameter of the observable universe roughly 93 billion light-years; universe age about 13.8 billion years).
