Dark matter does not do much in the universe, but it had special importance in the early universe. It was the main source of heat for new stars.
The concepts related to dark matter are quite boring in nature and one thing is common here, that is gravity.
We know the effect of dark matter on galaxies and other large structures in the universe only through microgravity. Dark matter is of passive nature.
It does not react with light, with ordinary matter or with itself. It's like you're swimming in a vast ocean of dark matter and you don't even know it.
But this simplified picture of dark matter has some problems. When astrophysicists use computer simulations to construct galaxies including dark matter, they find that dark matter is boring and beyond reality.
It may also be that dark matter is of a slightly complex nature, or it may be that it still does not interact with light or normal matter, but perhaps does occasionally react with itself.
However, this self-interaction may not be very strong, otherwise the dark matter would have been bundled up into tiny balls or would have been annihilated long ago.
This "reaction but not much" hypothesis makes it challenging for astronomers to come up with ways to test the scenario.
The very early universe, when it was only two hundred million years old, was vastly different than it is today.
For one, it was much denser, as all the material in the universe was confined to a very small amount.
Second, it was very deep, because stars and galaxies had not yet formed.
At that time, the universe was made up of dark matter and neutral hydrogen and helium molecules.
Gradually, over the course of eons, all that material began to collapse gravitationally, creating ever larger structures.
The first protostars began as dense clusters no larger than about one-thousandth the size of the Sun.
In the traditional picture of the formation of the first stars, those clumps consistently grew to be a hundred times the size of the Sun, powered by nuclear fusion in their cores.
But a team of astrophysicists has realized that the traditional story may be different, as they recently told in a new paper appearing in the preprint journal arXiv.
If dark matter interacts with itself, then when dark matter particles collide they release a little bit of energy.
Not every collision produces much, but there may have been enough density to eliminate dark matter to become a major player in the sites of star formation in the early days of the universe.
In this scenario, the first stars are not powered by nuclear fusion, but instead annihilate the dark matter in their cores.
The research team calls them "dark stars," even though the stars themselves are still mostly made of normal matter.
These stars do not exist in the modern universe, because the density of dark matter is so low, so we cannot see them in galaxies today.
But researchers are hopeful that the James Webb Space Telescope (JWST), specifically designed to study the early universe and the formation of the first stars. It will reveal new information related to these dark stars.