Why is it that scientists think that dark matter annihilates

[rmoh13]

Why do scientists think that dark matter annihilates just like antimatter? How is it that dark matter during annihilation can produce light when it cannot emit or absorb light itself?

 

[newjerseyrunner]

They don't, in order for it to exist this long, it should not annihilate in any way. It might decay, but that's not the same. Just because something doesn't interact with light doesn't mean that it can't produce it. All interactions in the universe must obey the law that energy can not be created or destroyed, so if you have a 10TeV particle decaying into 2 4.8TeV particles, the universe must use that extra .4TeV for something, if there is no stable particle at that size, it's emitted as one or two photons.

 

[Orodruin]

They don't, in order for it to exist this long, it should not annihilate in any way. It might decay, but that's not the same.

This is backwards. It is much easier for dark matter to have significant annihilation when you collect large amounts of it than for it to have significant decay. Also, we do not know that it annihilates, but it is a feature of many popular dark matter models.

Just because something doesn't interact with light doesn't mean that it can't produce it.

Yes it does. Dark matter producing light in annihilations or decays are generally doing so in suppressed processes or by the secondary interactions of the annihilation or decay products.

All interactions in the universe must obey the law that energy can not be created or destroyed, so if you have a 10TeV particle decaying into 2 4.8TeV particles, the universe must use that extra .4TeV for something, if there is no stable particle at that size, it's emitted as one or two photons.

You can easily take care of this by giving the produced particles more kinetic energy. There is no a priori need to emit additional photons. If the produced particles are charged, there is however a possibility that this occurs.

 

[Chalnoth]

Why do scientists think that dark matter annihilates just like antimatter?

Dark matter is likely made up of nearly equal parts matter and anti-matter. The dark matter anti-particles would naturally annihilate with the particles, though obviously this effect has to happen quite slowly in order to allow so much dark matter to remain after billions of years.

How is it that dark matter during annihilation can produce light when it cannot emit or absorb light itself?

During annihilation (or decay), dark matter could emit charged particle/anti-particle pairs which do emit light.

 

[Orodruin]

Dark matter is likely made up of nearly equal parts matter and anti-matter.

This is not really true. In many models, dark matter is a Majorana fermion, meaning that it is its own antiparticle. If this is the case then there is no way of defining anti dark matter which is different - it is still going to annihilate though.

In addition, there are models of asymmetric dark matter which became popular a few years back. In most of those, dark matter does not decay.