Primary
There were no galaxies, stars, or planets for the first 150 million years following the Big Bang. The universe was devoid of features.
The first stars appeared as time passed. Galaxies formed when stars collided. Galaxies began to form clumps. The galaxies and all the material in between the galaxies make up those clusters. Clumps of matter collided, and our solar system’s planets began to form around the sun.
Our solar system, galaxies, and clusters of galaxies must all be held together by something. Gravity is the “glue” that holds everything together.
The space between galaxies in some clusters is filled with gas so hot that scientists can’t see it with visible light instruments. Only the gas can be seen.
Vera Rubin and her colleagues from the United States confirmed this result in the 1970s by looking at galactic rotation. They also discovered that individual galaxies, not just clusters, have more mass than what the naked eye can see. The discoveries of Rubin and her colleagues contributed to the acceptance of the concept of dark matter.
Scientists know a lot more about what dark matter isn’t than what it is, though they do have some ideas about what it could be.
Brown dwarfs, or “failed” stars that never ignited due to a lack of mass, could constitute dark matter. White dwarfs, the nuclei of tiny to medium-sized stars that have died, could be dark matter.