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 which allows scientists to postulate that they can make the large missing energy in the universe known as dark matter. Second, they are collision-less which is consistent with the properties of dark matter. The axion theory attracts scientists because it can solve two enigmas at once, making it more likely to be true. The axion theory 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 make 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.
Rejecting the premises