# Distribution of Dark Matter around galaxies

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 ?

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one plausible explanation is that dark matter was not "dark" since the conception of universe! According to what i heard about dark matter, they are black holes and neutron stars which were present as ordinary matter. But then this also doesn't explain your answer... :( –  Vineet Menon Sep 29 '11 at 12:09
The theory of dark matter as neutron stars and black holes has, for the most part, been shown not true. Look at the EROS, MACHO, and OGLE studies. –  Benjamin Horowitz Sep 29 '11 at 12:38