How much matter is there in the universe? Now we (kind of) know

According to the team’s result, the universe consists of (31.5% matter and 68.5% dark energy).

“We have succeeded in making one of the most precise measurements ever made using the galaxy cluster technique,” said astronomer Gillian Wilson of UC Riverside.

What the universe is made of. Credits: UCR/Mohamed Abdullah.

In the broadest and non-technical sense, the universe seems to consist of three things: dark energy, dark matter, and ‘normal’ matter. Normal matter is everything we see and know — your desk, your phone, the Earth, the Sun, and so on. Dark energy and dark matter… we’re not really sure what they are and we’ve not really observed them directly, but we have seen their effects.

As is often the case in astrophysics, more is unknown than is known. For now, let’s just say that dark and normal matter can be loosely grouped into one category we’ll call matter (we won’t get into what dark matter is, that’s a whole new can of worms).

Dark energy is, for the lack of a better term, not matter. As far as we can tell, the universe is not only expanding, but it’s expanding at an accelerated rate. This dark energy is what seems to be driving this accelerated expansion — we don’t know what it is, we just know that it’s expanding the universe. It’s weird but you’ll have to live with that explanation because, well, we don’t have a better one yet.

Understanding the magnitude of dark energy is actually crucial to our understanding of the Universe. Even if we don’t know exactly what it is, knowing how it affects the universe expansion is essential. So how do you measure something when you’re not even sure what it is?

Turns out, galaxy clusters are a good tool for that. Galaxy cluster (neighborhoods of galaxies). Galaxy clusters are made of matter that has come together under the pull of gravity, and the number of clusters we can observe in a volume of space is dependent on the amount of matter. So if by counting and measuring galaxy clusters we can get a decent idea about the matter in the universe, and everything else is dark energy.

Simulations of galaxy cluster formations with different amounts of matter. By checking which version most closely matches real observations, astronomers can figure out the most likely amount of matter in the universe. Image credits: UCR/Mohamed Abdullah

Of course, this is more easily said than done.

“The ‘Goldilocks’ challenge for our team was to measure the number of clusters and then determine which answer was ‘just right’. But it is difficult to measure the mass of any galaxy cluster accurately because most of the matter is dark so we can’t see it with telescopes,” said astronomer Mohamed Abdullah of the University of California, Riverside and the National Research Institute of Astronomy and Geophysics in Egypt.

Still, after a close analysis and a comparison with computer simulation, Abdullah and colleagues managed to create what they believe to be the most accurate measurement of matter in the universe. Spoiler alert: it’s… not that much.

“To put that amount of matter in context, if all the matter in the Universe were spread out evenly across space, it would correspond to an average mass density equal to only about six hydrogen atoms per cubic meter,”

“However, since we know 80 percent of matter is actually dark matter, in reality, most of this matter consists not of hydrogen atoms but rather of a type of matter which cosmologists don’t yet understand.”

So in reality, you’d get about one atom of matter per cubic meter — almost nothing. But this is the universe as we know it. The planets and the stars, you and me, we’re just one atom per cubic meter to the universe. As for the rest, we’re just now starting to understand what it is.

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