Main Theories for the Formation and Evolution of Galaxies

The Big Bang theory suggests that all matter and energy in the universe originated from a singularity approximately 13.8 billion years ago.
An ever-expanding universe began as a very hot, dense point nearly 13.8 billion years ago. This ‘Big Bang’ was not located in space, as space itself expanded along with time from this origin.

The earliest galaxies are thought to have formed via gravitational collapse of slightly denser regions in the universe, following the initial expansion.
These areas became increasingly dense over time, drawing nearby matter with their increasingly stronger gravity, ultimately creating galaxies and stars.
Cosmic Microwave Background Radiation (CMBR), a radiation that permeates throughout the universe, is an important evidence supporting this theory.

Dark matter, an unobserved form of matter present throughout the universe, is believed to have played a crucial role in the structure formation after the Big Bang.
The presence of dark matter is inferred due to its gravitational effects on visible matter, radiation, and the structure of the universe.
Dark matter, although invisible and detectable only by its gravitational effects, is thought to compose approximately 85% of the total matter in the universe.
Dark matter would have interacted gravitationally with normal matter (baryonic matter), allowing galaxy formation to occur sooner than if there was no dark matter.

Over time, galaxies are observed to evolve and change, via processes such as star formation and supernovae, or by interacting with other galaxies.
Such interactions can manifest as galaxy collisions or mergers, which can cause drastic morphological changes and trigger new waves of star formation.
Galaxies may also interact less violently, distorting each other’s shape gravitationally without merging, or stripping matter away from one galaxy into another.

The future evolution of galaxies relies greatly on the density and composition of the universe, which are still topics of ongoing research.
If the density of the universe is higher than the critical density, it may eventually stop expanding and even start to contract, leading to a ‘Big Crunch’.
This would ultimately cause galaxies to merge together and increase in size until all matter collapses into a singularity.