Spacetime is the distribution of matter over time. It is made of matter. It is defined by Einstein's General Relativity equation. It is full of dark energy. It has 4 dimensions; three spatial and one time. It is our universe.I really don't know, please tell me what it is. What is it made of? How does it get distorted by mass? Is it empty or full of energy? How many dimensions does it contain? Is it in our universe or is our universe in it?
It's amazing the lengths you will go to to ignore the universe being created from matter that was not pre-existing. You think asking all of those questions excuses your dismissal of the CMB and e=mc^2.
Atheists profess a love of science but shit all over it when science does not suit their preconceived ideas. The universe began. It popped into existence and then expanded and cooled. It was not created from existing matter. That's the science. Deal with it.