As clumps containing cold dark matter and normal matter collapse, they heat up. When a clump collapses to about half its maximum size, the increased thermal motion of particles tends to inhibit further collapse. Whereas normal matter can overcome this effect and continue to collapse, dark matter cannot. Explain the reason for this difference.

Thermal motion is the random movement of particles within a substance due to their thermal energy. Imagine particles inside a clump that is collapsing. As the clump's size decreases, the density increases, causing particles to move faster.
In essence, thermal motion increases as particles get closer to each other. This increase in motion generates heat, which can create a barrier to further collapse of the matter. Understanding thermal motion is essential to grasp why normal matter can continue to collapse while dark matter cannot.

Radiative cooling refers to the process through which normal matter can lose energy in the form of radiation. When a clump of normal matter collapses, it heats up due to increased thermal motion.
However, gases and dust within the clump can emit this generated heat as electromagnetic radiation, helping them to cool down.
This cooling mechanism allows normal matter to overcome the resistance posed by thermal motion and continue collapsing. In contrast, since dark matter cannot undergo radiative cooling, it remains heated and ultimately halts its collapse.

Electromagnetic interactions are the forces that occur between charged particles. These interactions play a crucial role in the behavior of normal matter. Atoms and molecules of normal matter experience electromagnetic forces, allowing them to emit radiation.
This radiative process is vital for cooling down normal matter as it collapses. Dark matter, however, interacts very weakly, if at all, with electromagnetic forces.
Because of this, dark matter can't radiate away energy, which is why it cannot cool down and overcome the increased thermal motion during collapse.

Normal matter and dark matter behave differently during the collapse of clumps. Normal matter includes protons, neutrons, and electrons, which are influenced by electromagnetic interactions. This allows normal matter to radiate away energy through processes such as radiation.
As a result, normal matter can cool down, overcoming the barrier posed by increased thermal motion, and continue to collapse.
Dark matter, on the other hand, is a type of matter that does not interact significantly with electromagnetic forces. It can't radiate away the heat generated by thermal motion. Therefore, dark matter clumps are unable to continue collapsing once they heat up, ending their contraction prematurely.