The inclusion of a period of ( effective ) matter domination following inflation and prior to the onset of radiation domination has interesting and observable consequences for structure growth .
During this early matter-dominated era ( EMDE ) , the Universe was dominated by massive particles , or an oscillating scalar field , that decayed into Standard Model particles , thus reheating the Universe .
This decay process could also be the primary source of dark matter .
In the absence of fine-tuning between the masses of the parent and daughter particles , both dark matter particles and Standard Model particles would be produced with relativistic velocities .
We investigate the effects of the nonthermal production of dark matter particles with relativistic velocities on the matter power spectrum by determining the resulting velocity distribution function for the dark matter .
We find that the vast majority of dark matter particles produced during the EMDE are still relativistic at reheating , so their free streaming erases the perturbations that grow during the EMDE .
The free streaming of the dark matter particles can also prevent the formation of satellite galaxies around the Milky Way and the structures observed in the Lyman- \alpha forest .
For a given reheat temperature , these observations put an upper limit on the velocity of the dark matter particles at their creation .
For example , for a reheat temperature of 10 MeV , dark matter must be produced with a Lorentz factor \gamma \lesssim 550 .