If you look up into the night sky with a telescope and look beyond what's visible to the naked eye, you'll notice a lot of stars that aren't real. Many of those bright spots are galaxies, which are collections of millions to trillions of stars. Galaxies are made up of stars, dust, and dark matter, which are all held together by gravity.
Galaxies are some of the most fascinating and mysterious objects in the universe. They come in a variety of shapes and sizes, from small, irregular galaxies to massive, spiral galaxies like our own Milky Way. But how are these incredible structures formed?
The dominant theory holds that galaxies form through a process of hierarchical structure formation, in which small structures merge to form larger ones. This process begins with small fluctuations in the density of matter in the early universe. These fluctuations eventually collapse under their own gravity to form dense, dark matter halos. Normal matter (such as gas and dust) begins to collect and collapse inside these halos, forming the first stars and galaxies.
These small galaxies continue to merge and grow over time, eventually forming larger and larger structures. As time passes, the central regions of galaxies become denser and denser, with more and more stars forming. The Milky Way is thought to have formed in this manner, with a dense central bulge surrounded by a flat disk of stars and gas.
The presence of dark matter is another important factor in the formation of galaxies. Dark matter is an enigmatic substance that accounts for approximately 85% of all matter in the universe but does not emit or absorb light, rendering it invisible to telescopes. The gravitational effects it has on normal matter, however, indicate its presence. Dark matter is thought to play an important role in the formation and evolution of galaxies because its gravity helps to hold galaxies together and shape their structures.
In addition to these processes of hierarchical structure formation, there are other mechanisms that can lead to the formation of galaxies. Galaxy collisions and mergers, for example, can cause bursts of star formation and change the shape and structure of the galaxies involved.
Overall, galaxies form in a complex and ongoing process that is still not fully understood. Astronomers continue to investigate galaxies' properties and structures in order to better understand how they form and evolve.
Astronomers don't know exactly how galaxies form. After the Big Bang, space was almost entirely made up of hydrogen and helium. Some astronomers believe that gravity drew dust and gas together to form individual stars, which then drew closer together to form collections that eventually became galaxies. Others believe that the mass of what would become galaxies drew together prior to the formation of stars within them. Astronomers are also improving their methods for measuring the mass of individual galaxies, as evidenced by this 2018 study that used the three-dimensional movements of several galaxies to narrow down the Milky Way's mass.
Finally, galaxy formation is a complex process that begins with small fluctuations in the density of matter in the early universe. These fluctuations eventually collapse under their own gravity, forming dense, dark matter halos that attract normal matter to form the first stars and galaxies. As small galaxies merge and grow, larger and larger structures, such as spiral galaxies, emerge. Dark matter is also important in the formation of galaxies because it helps to hold them together and shape their structures.