General relativity, Einstein's last and greatest concept, tells us that gravity isn't really a force at all. It's a trick of perception, like an optical illusion, only it isn't just optical.
We are three dimensional beings. That doesn't mean just our bodies, it's our minds too.
As mentioned earlier, physics tells us our universe must have more than three dimensions. How do we know this if we can't see it? Because the physics becomes simpler and more powerful in higher dimensions. What this means is that by assuming higher dimensions, physics makes more powerful predictions about how ordinary matter/energy around us behaves, and those predictions turn out to be deeply true and can be used to do things and build stuff that couldn't be done or built in the same way if they weren't true.
The trick of perception that "creates" gravity for us is the result of three-dimensional beings living in (at least) a four-dimensional universe which is curved in the 4th dimension. The amount by which it curves at any place is directly related to the amount of mass at that place.
Once again, it all seems to come back to mass. We still don’t know where it comes from or what it really is. (The famous Higgs boson is connected with one way of answering these questions, but I don’t think that type of answer interferes with this line of thinking. At some point we always seem to run into the “it’s turtles all the way down” problem*.)
Here’s what I have concluded so far:
Mass is always in motion (all mass has momentum).
You can’t have motion without mass because something has to move
You can’t have motion without space because it has to move somewhere else
You can’t have motion without time because it takes time to get somewhere else
You can’t have space without time, because anything has to have a duration in order to exist
You can’t have time without motion because motion is all we know of time.
You can’t have space without motion because you can’t have space without time.
Space, mass, motion, and time seem to be so intimately connected that one cannot be said to be more primal than the other. The four together appear as momentum. Time implies motion which requires mass and space. Mass increases with motion. Space and time decrease with motion. Mass + time + space equals momentum. Momentum is like Shiva--creator, preserver, and destroyer.
So the mass question can now be restated: where did momentum come from?
The outlier that may shed light on this is light.
*The reference is to a story of someone contending that the earth sits on the backs of a series of tortoises. Upon being challenged by the question “what does the last tortoise stand on?” the answer was “it’s turtles all the way down”.