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I was wondering if black holes are the greatest force of gravity how does other highly spoken about concepts such as dark energy or dark matter compare. Can a black hole absorb dark energy or dark matter? I though dark matter held galaxies together. If a black hole could absorb the dark matter would that be a problem: i.e. the universe getting smaller?

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Dark matter can, and probably does, fall into black holes and affects them just as any other form of matter or energy falling into them does. It doesn't fall in a huge amount because, like anything else approaching a black hole it usually isn't aimed straight in, but rather misses a bit and swings by and escapes again. Unlike normal matter, dark matter experiences almost no friction or collisions from other stuff near the black hole (it goes straight through everything, including itself, we think) so it doesn't get caught in the accretion disk or anything. Dark matter falling into the central SMBH doesn't do much to galaxy, because its mass is still there pulling on the stars and so on just as it did before it fell in.

Dark energy is another story. In most of the various competing theories in which it exists, it is an energy inherent in space. Every cubic meter of space contains a certain (small) amount of dark energy, which can't move around or anything. It shows up as something like a pressure, causing space to tend to expand whenever there isn't a larger force (like the gravity of nearby masses) causing it to contract.

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  • $\begingroup$ Critically, the density of dark matter (assuming it and not modified gravity is the correct explanation for these phenomena) is very low - the mass of a squirrel in the volume of the sun is a useful order of magnitude statistic. So, any impact of dark matter on a black hole would be negligible relative to its mass. Dark energy, assuming it is a substance, is even more thinly spread and assuming is a default space-time curvature, is tiny of the volume/distance scales of even a supermassive black hole. $\endgroup$ – ohwilleke Aug 27 at 20:24
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Dark matter can certainly fall into a black hole. Nobody is sure what dark matter is, but everyone agrees that its gravitational effects on galaxies prove its existence. One popular but very speculative idea is that it is made of Weakly Interacting Massive Particles (WIMPS) which hardly ever interact with normal matter, but so far particle physicists have never detected or isolated a WIMP. One thing that everybody accepts about them is that they interact gravitationally. That being the case, they would have no option but to be drawn into a black hole if they were very close to it. As, like neutrinos, they seldom interact with matter and there would be no friction or compression resulting from them, they would not brighten the accretion disc, but when they entered the black hole they would increase its mass.

As for dark energy, not everyone is convinced of its existence. It is said to make the space between the galaxies expand faster than was the case very long ago, but no one knows how it does this or what kind of particle could carry this energy. Further data may yet indicate that dark energy is an illusion, but as things stand it is thought that this speeding up of the expansion will prevent gravity eventually bringing the expansion to a halt and starting a collapse, so he universe will go on expanding for ever..

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