The Composition of the Universe

Stars and galaxies, such as the Andromeda Galaxy and our own Milky Way, only make up a tiny part of the mass of the universe. What else is there?. Adam Evans/Wikimedia Commons.

The universe is a vast and fascinating place. When astronomers consider what it's made of, they can point most directly to the billions of galaxies it contains. Each of those has millions or billions—or even trillions—of stars. Many of those stars have planets. There are also clouds of gas and dust. 

In between the galaxies, where it seems there would be very little "stuff", clouds of hot gases exist in some places, while other regions are nearly empty voids. All that is material that can be detected. So, how difficult can it be to look out into the cosmos and estimate, with reasonable accuracy, the amount of luminous mass (the material we can see) in the universe, using radio, infrared and x-ray astronomy?

Detecting Cosmic "Stuff"

Now that astronomers have highly sensitive detectors, they are making great advances in in figuring out the mass of the universe and what makes up that mass. But that's not the problem. The answers they're getting don't make sense. Is their method of adding up the mass wrong (not likely) or is there something else out there; something else that they can't see? To understand the difficulties, it's important to understand the mass of the universe and how astronomers measure it.

Measuring Cosmic Mass

One of the greatest pieces of evidence for the mass of the universe is something called the cosmic microwave background (CMB). It's not a physical "barrier" or anything like that. Instead, it's a condition of the early universe that can be measured using microwave detectors. The CMB dates back to shortly after the Big Bang and is actually the background temperature of the universe. Think of it as heat that is detectable throughout the cosmos equally from all directions. It's not exactly like the heat coming off the Sun or radiating from a planet. Instead, it's a very low temperature measured at 2.7 degrees K. When astronomers go to measure this temperature, they see small, but important fluctuations spread throughout this background "heat". However, the fact that it exists means that the universe is essentially "flat". That means it will expand forever.

So, what does that flatness mean for figuring out the mass of the universe? Essentially, given the measured size of the universe, it means there has to be enough mass and energy present within it to make it "flat".The problem? Well, when astronomers add up all of the "normal" matter (such as stars and galaxies, plus the gas in the universe, that's only about 5% of the critical density that a flat universe needs to remain flat.

That means that 95 percent of the universe hasn't yet been detected. It's there, but what is it? Where is it? Scientists say that it exists as dark matter and dark energy

The Composition of the Universe

The mass that we can see is called "baryonic" matter. It is the planets, galaxies, gas clouds, and clusters. The mass that can't be seen is called dark matter. There is also energy (light) that can be measured; interestingly, there's also the so-called "dark energy." and nobody has a very good idea of what that is. 

So, what does make up the universe and in what percentages? Here's a breakdown of the current proportions of mass in the universe.

Heavy Elements in the Cosmos

First, there are the heavy elements. They make up about ~0.03% of the universe. For nearly half a billion years after the birth of the universe the only elements that existed were hydrogen and helium They aren't heavy.

However, after stars were born, lived, and died, the universe started getting seeded with elements heavier than hydrogen and helium that were "cooked up" inside stars. That happens as stars fuse hydrogen (or other elements) in their cores. Stardeath spreads all those elements to space through planetary nebulae or supernova explosions. Once they are scattered to space. they are prime material for building the next generations of stars and planets. 

This is a slow process, however. Even nearly 14 billion years after its creation, the only a small fraction of the mass of the universe is made up of elements heavier than helium.


Neutrinos are also part of the universe, although only about 0.3 percent of it. These are created during the nuclear fusion process in the cores of stars, neutrinos are nearly massless particles that travel at nearly the speed of light. Coupled with their lack of charge, their tiny masses mean that they do not interact readily with mass except for a direct impact on a nucleus. Measuring neutrinos is not an easy task. But, it has allowed scientists to get good estimates of nuclear fusion rates of our Sun and other stars, as well as an estimate of the total neutrino population in the universe.


When stargazers peer out into the night sky most of what the see is stars. They make up about 0.4 percent of the universe. Yet, when people look at the visible light coming from other galaxies even, most of what they see are stars. It seems odd that they make up only a small part of the universe. 


So, what's more, abundant than stars and neutrinos? It turns out that, at four percent, gases make up a much bigger part of the cosmos. They usually occupy the space between stars, and for that matter, the space between whole galaxies. Interstellar gas, which is mostly just free elemental hydrogen and helium makes up most of the mass in the universe that can be directly measured. These gases are detected using instruments sensitive to the radio, infrared and x-ray wavelengths.

Dark Matter

The second-most-abundant "stuff" of the universe is something that no one has seen otherwise detected. Yet, it makes up about 22 percent of the universe. Scientists analyzing the motion (rotation) of galaxies, as well as the interaction of galaxies in galaxy clusters, found that all of the gas and dust present is not enough to explain the appearance and motions of galaxies. It turns out that 80 percent of the mass in these galaxies must be "dark". That is, it's not detectable in any wavelength of light, radio through gamma-ray. That's why this "stuff" is called "dark matter". 

The identity of this mysterious mass? Unknown. The best candidate is cold dark matter, which is theorized to be a particle similar to a neutrino, but with a much greater mass. It is thought that these particles, often known as weakly interacting massive particles (WIMPs) arose out of thermal interactions in early galaxy formations. However, as yet we have not been able to detection dark matter, directly or indirectly, or create it in a laboratory.

Dark Energy

The most abundant mass of the universe is not dark matter or stars or galaxies or clouds of gas and dust. It's something called "dark energy" and it makes up 73 percent of the universe. In fact, dark energy isn't (likely) even massive at all. Which makes its categorization of "mass" somewhat confusing. So, what is it? Possibly it's a very strange property of space-time itself, or maybe even some unexplained (so far) energy field that permeates the entire universe. Or its neither of those things. Nobody knows. Only time and lots and lots more data will tell.

Edited and updated by Carolyn Collins Petersen.


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Millis, John P., Ph.D. "The Composition of the Universe." ThoughtCo, Apr. 5, 2023, Millis, John P., Ph.D. (2023, April 5). The Composition of the Universe. Retrieved from Millis, John P., Ph.D. "The Composition of the Universe." ThoughtCo. (accessed June 8, 2023).