Cosmologists like to know how fast the universe is expanding. To find this out they used Hubble's Law, which tells us how fast galaxies are receding from ours based on how far away from us those galaxies are.
The figure the standard cosmological model predicts is about 67 km/s/Mpc. That means that for each additional megaparsec a galaxy is away from us, it is receding from us by an extra 67 km/s.
A megaparsec is a distance scale used in astronomy and you can see how it works by using your finger. Close one eye, hold your finger up quite close to your face and align it with something on a wall in the room you're in. Keep your finger there and switch which eye is closed. You will see the object your finger's aligned with will appear to shift depending on which eye is open. Repeat the experiment but this time hold your finger further away from your face, at arms length. You will notice that the closer your finger is to your face, the more it will appear to move relative to the point on the wall.
That's called parallax and you can see how it might be used to measure distance. Astronomers need a bigger measurement than the space between their eyes, so they used the Earth's orbit around the Sun. They will look at the star they're trying to measure (equivalent to your finger) in relation to the background stars (equivalent to the wall you were looking at). They'll measure it in January, say, and then again July when the Earth's on the opposite side of the Sun. This is how they get their parallax.
If the angle between the target star and the background star changes by one arc second (1/3600 of a degree), they call the resulting distance a parsec. It's equivalent to about 19 trillion miles (31 trillion kilometres or 3.3 light years). A megaparsec is a million parsecs.
This works well for stars that are relatively close to us and it is the first rung on the measurement ladder astronomers use to determine distances in the universe. It doesn't work so well for stars a long way from us because the shift in the angle gets smaller and smaller and becomes hard to measure. They use other methods to determine those distances, but those other methods are all built on top of the parallax baseline.
So, in order to determine how fast the universe is expanding, they need to know how far away things are, and it all ultimately depends on how accurate their parallax measurements are.
The European Space Agency's Gaia spacecraft has spent the last six years measuring the parallaxes of 1.3 billion stars, which is a big help to astronomers and the accuracy of their measurements.
What they have determined, based in part on these more accurate parallax measurements, is that the universe is actually expanding at 73.2 km/s/Mpc. They've done that within a margin of error of just 1.8%, which means the predicted figure of 67 km/s/Mpc cannot be correct.
So there's an unknown quickening agent at work, making the universe expand faster than it should be. This is on top of dark energy, which is already accounted for in the model.
At about 4500 megaparsecs (15 billion light years) galaxies are receding from us faster than light. This does not break the light speed limit, though. That's because those galaxies are not travelling faster than light through space, it's space itself that's expanding.
Do not under any circumstances lend money to beings further than 4500 Mpc away because you'll never see them again.