How much matter is there in the universe? It’s a mind-boggling question, and involves a daunting amount of maths – but California scientists say they have figured it out.
In total, matter – the stuff from which humans and objects on Earth are made – makes up around a third of the total matter and energy in the universe.
If it was spread out across the space in the universe, it would mean just six hydrogen atoms per cubic metre, says study author Mohamed Abdullah, a graduate student at the University of California, Riverside.
The research was published in the Astrophysical Journal.
Read more: There might once have been life on the moon
But 80% of that is dark matter, a mysterious form of matter which is still not fully understood, says Abdullah.
The invisible ‘dark matter’ is so called because it emits no light, and is extremely difficult to measure.
Abdullah said: “Since we know 80% 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.”
The rest of the total is made up of dark energy, a mysterious force thought to be driving the expansion of the universe, Science Alert reports.
The researchers “weighed” the matter in the universe by observing galaxy clusters, which have formed from matter that has collapsed over billions of years under its own gravity.
Comparing the number of galaxy clusters with the number from predictions from numerical simulations is a good way to work out the amount of matter, the researchers say.
“A higher percentage of matter would result in more clusters,” Abdullah said. “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.”
To overcome this difficulty, the UCR-led team of astronomers first developed GalWeight, a cosmological tool to measure the mass of a galaxy cluster using the orbits of its member galaxies.
“We have succeeded in making one of the most precise measurements ever made using the galaxy cluster technique,” said coauthor Gillian Wilson, a professor of physics and astronomy at UCR in whose lab Abdullah works.