Cosmologists say it's because the Big Bang occurred. So far I have no problem understanding them. Before the Big Bang there was nothing. As I understand it, the Big Bang resulted in particles and antiparticles of each sort in strict parity, so the total matter and energy in the universe is still zero. The only difference is that it now has organization, so it's in a state of lower entropy. The Second Law allows this: a local reversal of entropy. I think the problem we're running into here is neither scientific nor philosophical, but linguistic. The space-time continuum is an abstraction. Abstractions only "exist" in our minds so we need a different word. It's a coordinate system for identifying the location of its contents, so that we can talk about them meaningfully. All of the contents would still exist if we weren't here to define a space-time continuum. There would be no good way to analyze and study them, or even describe where they are. But that's okay because there would be no one around to analyze them and study them, so the abstraction of the S-T continuum would not be needed or missed.