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Is Dark Matter matter? [Q&A]


Karthik asks, if we convert some Dark Matter to energy, will that be energy as we know it, or Something Else™?

The history of astronomy has been a series of shocks to the ego of mankind, as the importance of our place in the Universe keeps shrinking. First it turned out that the Earth was not the center of the solar system, and then that the Sun was not the center of the Galaxy, and then that the Galaxy was just an average speck in a much larger Universe.

Similarly, we used to think that what the Earth was made of — rock, dirt, trees, animals — was what the Universe was made of. Then we discovered that all this is a miniscule fraction compared to how much Hydrogen and Helium there is. But even this is still matter that can be seen, detectable through the radiation they emit.

But it turns out, if you just count up how much matter you can see, there just isn’t enough to make sense of the Universe. There is a ton of stuff in galaxies and clusters of galaxies that cannot be seen, but clearly exerts gravitational pull on the stuff that can be seen. What is this dark material?

We don’t really know. There could be a lot of stuff locked up in brown dwarfs, planets, comets, etc. (collectively called Massive Compact Halo Objects, or MACHOs), but there don’t seem to be enough of them. They could be exotic particles that have no charge and only interact with other particles via gravity (collectively called Weakly Interacting Massive Particles — WIMPs). There is nothing to indicate (yet) that these are not particles that don’t fit into standard models of particles physics. So, at least in terms of mass-energy equivalence, they are normal particles: they have kinetic and gravitational potential energy, and in the case of elementary particles, they can annihilate with their corresponding anti-particles to produce photons.