Drive by science

[Dragar]

Originally Posted by JoeP

HOW CAN THERE BE GAPS IN SPACE?

It's really best not to think of this way at all.

When you write down Einstein's equations for gravity, you have one term that describes the curvature of space time, and one term that describes the combined energy-momentum of matter. So energy/momentum bends spacetime - it gravitates. (The addition to this equation is that the curvature of spacetime describes the way matter moves through it - matter tells spacetime how to curve, and the curvature of spacetime tells matter how to move.)

Now, there's a weird thing where isn't the most general form of the field equations. There's another, perfectly consistent form, which is where you have an extra term - a constant (I'm sloppily describing the mathematics here, but it's good enough).

For small values of that constant, it's pretty undetectable in everyday life. And it can live on either side of that equation. It can be matter, or it can be curvature.

This term, it turns out, perfectly describes the accelerating expansion of the universe - what's now termed 'dark energy'. On large scales (for for a particular sign, depending on which side of the equation it is) it functions as repulsion between matter.

But because we don't have a physical interpretation for this mathematics, you can either view it as some sort of matter with a very odd behaviour (you can't ever 'dilute' it - if you consider it in more space, there's just more of it in terms of its gravitational effect, so the energy density is always the same) or as a sort of intrinsic addition to the curvature of even empty spacetime. Really, we don't know what it is.

Also, space doesn't expand. Stuff gets further apart.

Now, that's dark energy. Dark matter is much more boring. It's just matter that doesn't glow (hence 'dark'). The dark energy was discovered later and seemed to acquire the same moniker, but it's really very different and probably needs a better name (I like Sean Carrol's 'smooth tension', but it never really caught on).