It turns out that dark matter makes up at least 27% of the universe. Everything on earth only adds up to less than 5% of the universe. Dark matter has been mysterious for a long time and continues to be in so many aspects. So what is dark matter and what do we know about it?
Dark matter is made of non-Baryonic matterShow moreShow less
Dark matter is not baryonic at all. It is made of more exotic particles like axions or Weakly Interactive Massive Particles (WHIMPs)
Scientists proposed axions in 1970s as small particles that help balance the neutrons and make them symmetrical. Because of their vast numbers, scientist proposed them as candidates of dark matter in the universe
Scientists postulated axions to account for the neutral charge and symmetry of neutrons in the nucleus. Neutrons are made of quarks which are smaller particles that have charge. Neutrons overall are neutral and have symmetrical distribution of charge. Scientists predicted that there must be an energy forcing the charges to distribute equally and leading to zero net charge on the neutrons. Particles like axions may account for the energy and solve this problem. 
Axions also fit the two criteria necessary to make up dark matter. First, there is a massive number of axions whcih allows scientists to postulate that they can make the missing energy in the universe, dark matter. Second, they are collison-less which is consistent with the properties of dark matter. The axion theory attracts scientists because it can solve two enigma at once, making it more likely. It explains why the universe is heavier than it looks and it explains the neutral charge of neutrons. Many scientists consider axions as good candidates for dark matter. 
It is difficult to conclude that axions are what makes up dark matter. Scientists have yet to detect axions. Axions are so light that they cannot be seen or be received on a detector. Also, they do not emit light that scientists can measure. They do not collide with normal matter but pass through it. Collision releases energy and quantifying the energy would indicate the presence of particles. Because of all of these obstacles, axions are still hypothetical particles.