Behind a moving source the frequency decreases. Ahead of the source the frequency increases.

In distant galaxies an elements spectrum would be shifted towards the red end because red means a longer wavelength and lower frequency. This means they are receding. This was discovered by Edwin Hubble.

The big bang was a massive explosion from a very small point (singularity)

If the density of the universe is less than a certain value then it will continue expanding forever.

If the density of the universe is more than a certain value then it will stop expanding and contract - the big crunch!

We're not really sure what it is. It could be a massive object like a black hole or brown hole. Or it could be tiny particles smaller than atoms that can pass through ordinary matter. They are both theories to explain the universe's missing matter?

This is the energy that causes the universe to expand as an ever increasing rate.

It makes the stars appear much brighter because it is wider than your eye so it collects more light. It also makes planets and moons appear larger and you can see more detail.

The atmosphere scatters the light and makes the images fuzzy.

They map out sources of radio-waves such as distant galaxies.

To carry detectors of electromagnetic waves that can't penetrate the earth's atmosphere. They can see massive stars that explode emitting gamma rays and black holes.