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Galaxies are vast systems that contain stars, star clusters, interstellar gas, dust, and dark matter, bound together by gravity. In the observable universe, there are billions of galaxies, each containing millions to trillions of stars. These galaxies come in different shapes and sizes, including spiral, elliptical, and irregular forms. When astronomers observe galaxies, they often look at their rotation speeds. These rotation speeds hold clues to the presence of dark matter.

The stars in galaxies rotate around their centers. Interestingly, the outer stars rotate almost as quickly as those near the center, which shouldn't happen if only visible matter was present. This curious rotation speed is a clue: the mass of the galaxy is much larger than what we can see.

• Spiral galaxies are known for their flat, rotating disks and spiral arms.
• Elliptical galaxies are more 3D and have older stars.
• Irregular galaxies do not have a definite shape.

Hence, the presence of hidden or dark matter is inferred from these observations.

Gravitational lensing is a fascinating phenomenon where light bends around massive objects, like galaxies or galaxy clusters, as it travels through space. This bending is due to the gravitational pull of these massive objects. According to Einstein’s General Theory of Relativity, gravity can warp spacetime, and massive objects create a sort of 'dent' around them, causing light to curve as it passes by.

When we look at gravitational lensing, we see distorted, magnified images of galaxies far away. The lensing happens because there's more mass than we can see, suggesting the existence of dark matter.

A few interesting points about gravitational lensing are:

• It allows astronomers to measure the mass of galaxies and clusters, including unseen dark matter.
• Strong lensing results in arcs and rings, like Einstein rings, where the alignment is perfect.
• Weak lensing is subtle but widespread and measured statistically.

Observing these patterns helps scientists infer the amount and distribution of dark matter in the universe.

The Cosmic Microwave Background (CMB) radiation is the faint glow of radiation left over from the Big Bang, which fills the entire universe. This radiation is crucial for understanding the very early universe and its subsequent evolution. As the universe expanded, this radiation cooled and spread out, eventually forming a uniform microwave signal we can detect today.

Studying the CMB provides information about the early universe's composition, including dark matter. Scientists look at fluctuations in the CMB's temperature to learn about the density and distribution of matter.

Key aspects of CMB:

• The CMB is almost uniform but has tiny fluctuations indicating varying densities.
• These fluctuations reveal the amounts of dark matter and ordinary matter.
• Satellites like the Planck observatory have mapped the CMB with high precision.

Through this detailed analysis, astronomers gain insights into the fundamental composition of the universe, confirming the presence of dark matter alongside visible matter.