Question

Dark Matter accounts for approximately 25% of the energy density of the Universe. The standard neutrinos account for the majority of this with only a small amount explained by a population of MACHOs. True or false? Explain.

Answer 1

yes dark matter accounts only for a quarter of the total energy density. It is mainly non-baryonic in nature. It is also thought that it may be consisting of yet to be discovered subatomic particles.

Massive astrophysical compact halo object is type of a body which can explain the presence of dark matter in galaxy halos.

Neutrinos can not account for even a significant portion of dark matter. so it is wrong to say that the standard neutrinos make up a majority of dark matter.

so the statement is false.

The main reason is that the neutrinos mass is insufficient to see the kind of gravitation that the dark matter can produce. Moreover, if neutrinos make up the dark matter, then it would be a hot-dark matter. But, we generally know that the dark matter is generally cold-dark matter.

If the dark matter were to be hot, and move quickly it would naturally prevent the gravitational growth of small scale structures to be streaming out of it. By the lyman-alpha forest we can rule out the possibility of dark matter being hot. so neutrinos don't really make up the dark matter.