Astronomers estimate that around 85% of all matter in the universe is dark matter, which means that only 15% of all matter is normal matter. Given dark energy, the name astronomers give to the accelerated expansion of the universe, black matter accounts for about 27% of all mass energy in the cosmos, according to CERN (opens in a new tab) (the European Organization for Nuclear Research).
Astronomers have a variety of tools to measure the total amount of matter in the universe and compare it to the amount of “normal” (also called “baryonic”) matter. The simplest technique is to compare two measurements.
The first measurement is the total amount of light emitted by a large structure, such as a galaxy, which astronomers can use to infer that object’s mass. The second measurement is the estimated amount of gravity needed to hold the large structure together. When astronomers compare these measurements on galaxies and clusters throughout the universe, they get the same result: there simply isn’t enough normal light-emitting matter to account for the amount of gravitational force needed. to hold these objects together.
Thus, there must be some form of matter that does not emit light: dark matter.
Related: The states of matter: definitions and phases
Different galaxies have different proportions of dark matter to normal matter. Some galaxies contain almost no dark matter, while others are almost devoid of normal matter. But measurement after measurement gives the same average result: about 85% of the matter in the universe does not emit or interact with light.
Not enough baryons
There are many other ways astronomers can validate this result. For example, a massive object, such as a cluster of galaxies, will distort the spacetime around it so much that it will bend the path of any light passing through it – an effect called gravitational lensing. Astronomers can then compare the amount of mass we see from light-emitting objects to the mass needed to account for the lens, again proving that extra mass must be hiding somewhere.
Astronomers can also use computer simulations to observe the growth of large structures. Billions of years ago, our universe was much smaller than it is today. It took time for stars and galaxies to evolve, and if the universe were to rely only on normal, visible matter, we wouldn’t see any galaxies today. Instead, the growth of galaxies required “pools” of dark matter for normal matter to accumulate in, according to a lecture by cosmologist Joel Primack (opens in a new tab)
Finally, cosmologists can go back to when the cosmos was only a dozen minutes old, when the first protons and neutrons formed. Cosmologists can use our understanding of nuclear physics to estimate the amount of hydrogen and helium produced at that time.
These calculations accurately predict the ratio of hydrogen to helium in the current universe. They also predict an absolute limit to the amount of baryonic matter in the cosmos, and those numbers agree with observations of galaxies and clusters today, according to astrophysicist Ned Wright (opens in a new tab).
Alternatives to dark matter
Alternatively, dark matter may be a misunderstanding of our theories of gravity, which are based on Newton’s laws and Einstein’s general theory. relativity.
Astronomers can modify these theories to provide explanations for dark matter in individual contexts, such as the motions of stars in galaxies. But alternatives to gravity have not been able to explain all observations of dark matter across the universe.
All the evidence points to dark matter being some kind of unknown particle. It does not interact with light or normal matter and only makes itself known through gravity. In fact, astronomers believe there are trillions and trillions of dark matter particles streaming through you right now. Scientists hope to soon discover the identity of this mysterious component of the universe.
#universe #dark #matter