It is well-known that measurements of the velocity profile in galaxies are not compatible with Newtonian laws. A way to circumvent the problem is to assume that galaxies are surrounded by a spherical halo of Dark Matter. The density of mass of this Dark Matter can then be computed from the velocity profile with Newton's laws.
My question is the following: the formation of a galaxy involves essentially only the gravitational interaction. According to Newton (and General Relativity too), the trajectory of a particle does not depend on its mass. As a consequence, I do not understand how usual matter and Dark Matter could have followed different trajectories leading to such different mass distributions nowadays. Can somebody explain this (apparent) paradox ?
Answer
Because normal matter can radiate (and therefore lose) energy when it gets hot, it is able to collapse into more compact configurations, like the stars that form galaxies. The real hold up in forming a galactic disk, I believe, is angular momentum. Dark matter, on the other hand, has no efficient way of releasing energy, so it keeps moving around quickly on large orbits, which leads to a more spherical halo.
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