How Many Galaxies Exist in the Universe?

galaxy survey image.
This Hubble Space Telescope view reveals thousands of galaxies stretching back into time across billions of light-years of space. The image covers a portion of a large galaxy census called the Great Observatories Origins Deep Survey (GOODS). NASA, ESA, the GOODS Team, and M. Giavialisco (University of Massachusetts, Amherst)

How many galaxies are there in the cosmos? Thousands? Millions? More?

Those are questions that astronomers revisit every few years. Periodically they count galaxies using sophisticated telescopes and techniques. Each time they do a new "galactic census", they find more of these stellar cities than they did before.

So, how many are there? It turns out that, thanks to some work done using Hubble Space Telescope, there are billions and billions of them.

There could be up to 2 trillion...and counting. In fact, the universe is more vast than astronomers thought, too.

The idea of billions and billions of galaxies may make the universe sound much bigger and more populated than ever. But, the more interesting news here is that there are fewer galaxies today than there were in the early universe. Which seems rather odd. What happened to the rest? The answer lies in the term "merger". Over time, galaxies formed and merged with each other to form larger ones. So, the many galaxies we see today are what we have left after billions of years of evolution.

The History of Galaxy Counts

Back at the turn of the 19th century into the 20th, astronomers thought there was only one galaxy — our Milky Way — and that it was the entirety of the universe. They saw other odd, nebulous things in the sky that they called "spiral nebulae", but it never occurred to them that these might be very distant galaxies.

That all changed in the 1920s, when astronomer Edwin Hubble, using work done on calculating distances to stars using variable stars by astronomer Henrietta Leavitt, found a star that lay in a distant "spiral nebula". It was farther away than any star in our own galaxy. That observation told him that the spiral nebula, which we know today as the Andromeda Galaxy, was not part of our own Milky Way.

It was another galaxy. With that momentous observation, the number of known galaxies doubled to two. Astronomers were "off to the races" finding more and more galaxies. 

Today, astronomers see galaxies as far as their telescopes can "see". Every part of the distant universe seems to be chock full of galaxies. They show up in all shapes, from irregular globs of light to spirals and ellipticals. As they study galaxies, astronomers have traced the ways they have formed and evolved. They've seen how galaxies merge, and what happens when they do. And, they know that our own Milky Way and Andromeda will merge in the distant future. Each time they learn something new, whether it's about our galaxy or some distant one, it adds to their understanding of how these "large-scale structures" behave.

Galaxy Census

Since Hubble's time, astronomers have found many other galaxies as their telescopes got better and better. Periodically they would take a census of galaxies. The latest census work, done by Hubble Space Telescope and other observatories, continues to identify more galaxies at greater distances. As find more of these stellar cities, astronomers get a better idea of how they form, merge, and evolve.

However, even as they find evidence of more galaxies, it turns out that astronomers can only "see" about 10 percent of the galaxies they know are out there. What's going on with that?

Many more galaxies that can't be seen or detected with present-day telescopes and techniques. An astonishing 90 percent of the galaxy census falls into this "unseen" category. Eventually, they will be "seen", with telescopes such as the James Webb Space Telescope, which will be able to detect their light (which turns out to be ultra-faint and much of it in the infrared portion of the spectrum).

Fewer Galaxies Means Less to Light up Space

So, while the universe has at least 2 trillion galaxies, the fact that it used to have MORE galaxies in the early days may also explain one of the most intriguing questions asked by astronomers: if there's so much light in the universe, why is the sky dark at night?

This is known as Olbers' Paradox (named for the German astronomer Heinrich Olbers, who first posed the question). The answer may well be because of those "missing" galaxies. Starlight from the most distant and oldest galaxies may well be invisible to our eyes for a variety of reasons, including the reddening of light due to the expansion of space, the universe’s dynamic nature, and the absorption of light by intergalactic dust and gas. If you combine these factors with other processes that reduce our ability to see visible and ultraviolet (and infrared) light from the most distant galaxies, these could all provide the answer to why we see a dark sky at night.

The study of galaxies continues, and in the next few decades, it's likely that astronomers will revise their census of these behemoths yet again.