If you look at distant galaxies, they are moving away from us,

And the more distant galaxies are moving faster.

So really, it’s not really an explosion of galaxies away from us;

All galaxies are moving away from all the other galaxies.

Space itself is getting bigger.

But you would expect that the expansion of the universe is something that will slow down.

These galaxies have gravity and they attract each other

So you would expect that the expansion of the universe will diminish with time.

So it was a big surprise in 1998 when we realized that the galaxies were actually speeding up.

If you look at one galaxy, non-only it is moving away from us,

But tomorrow it will be moving away from us faster.

This is what we call the acceleration of the universe.

But we are not completely shocked,

We have a good theoretical model handed down to us

by Einstein.

The idea that even empty space has energy, what we call vacuum energy or dark energy.

And this vacuum energy pushes space itself and causes distant galaxies to move away from us

Faster and faster. That is where the acceleration comes from.

So theoretical cosmologists are trying to understand what could be going on.

Is it really vacuum energy? Is some variation of that?

Or is it even a modification of Einstein’s general relativity, our theory of gravity?

If vacuum energy is the right answer, the universe will continue to expand,

The galaxies will move away from us and the universe will gradually become a lonelier, darker,

emptier place.