Why dark matter can not collapse into a blackhole under it's own weight?
No. Black holes are purely gravitational phenomena. In fact, the ONLY known interaction dark matter is capable of being a part of is the gravitational interaction and would collapse into a black hole just like any other forms of matter. We wouldn't be able to distinguish a dark matter black hole from a normal, non-charged matter black hole
Okay, might have been a little wrong there. However, you seem to be talking about dark matter falling into black holes. What about the case for dark matter itself? Is dense dark matter "masses" capable of gravitational collapse?
To repeat the answer already given, yes, dark matter can collapse to form black holes. As a practical matter, however, it is more difficult. Without electromagnetic interactions, clumps of dark matter have no way to dissipate their kinetic energy and tend to remain uncollapsed.
No, I didn't mean to imply dark matter falling into normal black holes. It is completely general. A sufficiently massive cluster of dark matter should gravitationally collapse into a black hole.
As Bill K mentioned, this would be much tougher practically to achieve but the end result would be the same.
A hypothetical candidate for dark matter is the neutralino, a supersymmetric particle which is it's own antiparticle and if it was to come into too close a contact with other neutralinos, would annihilate, hence there may be an inbuilt mechanism that stops dark matter from interacting like regular matter. This annihilation is actually one method being used to detect the presence of dark matter-
Dark matter is believed to be essentially collisionless, meaning it does not 'clump' hence is, for all practical purposes, unable to contribute mass to baryonic clumping processes [e.g., stars, galaxies, black holes, etc.] It is, however, believed capable of providing significant gravitational assistance to baryonic matter clumping processes.
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