The observed rotation curves of galaxies show that the stars in the outer regions of galaxies rotate faster than expected based on the visible mass of the galaxy. This suggests that there is additional mass, or dark matter, present in galaxies that is not visible.

Dark matter is thought to play a crucial role in the formation of galaxies by providing the initial density perturbations that lead to the collapse of gas and the formation of stars. These density perturbations are generated by the gravitational instability of dark matter.

Dark energy is a mysterious force that is causing the expansion of the universe to accelerate. The exact nature of dark energy is unknown, but it is thought to be responsible for the observed acceleration of the expansion of the universe.

The existence of dark matter and dark energy challenges our understanding of gravity. The standard model of gravity, general relativity, does not explain the observed effects of dark matter and dark energy. This suggests that either general relativity needs to be modified or that there is a new force of nature that is responsible for the effects of dark matter and dark energy.

Indirect detection experiments are used to detect dark matter by searching for the products of dark matter annihilation or decay. These experiments typically look for gamma rays, neutrinos, or other particles that are produced when dark matter particles interact with each other or with ordinary matter.

The Bullet Cluster is a galaxy cluster that is undergoing a collision with another galaxy cluster. The collision has caused the gas in the two clusters to collide and heat up, producing a bright X-ray glow. The dark matter in the two clusters has not collided, however, and it continues to move through each other. This suggests that dark matter is not made up of ordinary particles, which would have collided with each other and slowed down.

The cosmic microwave background is the leftover radiation from the Big Bang, the event that created the universe. The CMB is a faint glow of radiation that fills the entire universe and is a key piece of evidence for the Big Bang theory.

The Hubble constant is the rate at which the universe is expanding. It is measured in kilometers per second per megaparsec, where a megaparsec is a unit of distance equal to 3.26 million light-years. The Hubble constant is an important parameter in cosmology and is used to estimate the age of the universe and the distance to distant galaxies.

The dark energy density parameter is the fraction of the total energy density of the universe that is made up of dark energy. It is typically denoted by the symbol (\Omega_\Lambda) and is a key parameter in cosmology. The value of (\Omega_\Lambda) is currently estimated to be around 0.7, which means that dark energy makes up about 70% of the total energy density of the universe.

The matter density parameter is the fraction of the total energy density of the universe that is made up of ordinary matter. It is typically denoted by the symbol (\Omega_\mathrm{m}) and is a key parameter in cosmology. The value of (\Omega_\mathrm{m}) is currently estimated to be around 0.3, which means that ordinary matter makes up about 30% of the total energy density of the universe.

The radiation density parameter is the fraction of the total energy density of the universe that is made up of radiation. It is typically denoted by the symbol (\Omega_\mathrm{r}) and is a key parameter in cosmology. The value of (\Omega_\mathrm{r}) is currently estimated to be very small, around 10^-4, which means that radiation makes up a negligible amount of the total energy density of the universe.

The curvature density parameter is the fraction of the total energy density of the universe that is made up of curvature. It is typically denoted by the symbol (\Omega_\mathrm{k}) and is a key parameter in cosmology. The value of (\Omega_\mathrm{k}) is currently estimated to be very small, around 0, which means that the universe is very close to being flat.

The cosmological constant is a constant that is added to the Einstein field equations to account for the effects of dark energy. It is typically denoted by the symbol (\Lambda) and is a key parameter in cosmology. The value of (\Lambda) is currently estimated to be very small, around 10^-122 m^-2, but it is responsible for the observed acceleration of the expansion of the universe.

The fate of the universe is determined by the values of the cosmological parameters, such as the dark energy density parameter and the matter density parameter. Current observations suggest that the universe is flat and that the dark energy density parameter is greater than the matter density parameter. This means that the universe will continue to expand forever, eventually reaching a state of heat death.