Concerning the early universe – if mass and energy are equivalent, then this would mean that these two entities would have always been equivalent, even before the existence of space time. Therefore, mass and energy must have pre-existed the universe. However, there must have been an event that kick-started the universe into action.
The Standard model is the big bang preceded by inflationary stage of the universe. Inflation leads to Baryogenesis which leads to a Big Bang Nucleo Synthesis and creation of particles from Baryons created in the inflationary stage of the universe.
Assuming that the big bang model is correct. The universe started in a Hot Dense state, like what it says on the telly, and cooled to the temperature it is today as evidenced by the CBR.
Earlier competing theories suggested particles are created constantly, but could not explain the CBR.
Evidence can be interpreted in different ways.