If matter and energy are the same thing, or at least transform into one another, which appeared on the scene first, matter or energy?
At the most fundamental level, they are the precisely the same thing.
What we think of as matter is composed of “particles,” but the particles are just excitations in quantum fields. Put energy in the field in a certain way and you get a particle. Put more energy in another way and you get the same particle traveling at a high speed, i.e. a particle with more energy.
This begs the question about the origin of the fields themselves. I don’t think physics can ever answer that question, though. They are probably just part of what the Universe is. Period.
Energy.
In the very earliest moments post Big Bang the temperature and pressure was WAY too high to allow matter to form. It took some time for the Universe to cool off before the energy soup could settle down into matter.