Why is the universe expanding? How can it do that?
Consider a gas balloon with painted dots on it. As you inflate the balloon, the dots move away from each other. In much the same way, galaxies are receding from the Earth, first described by Edwin Hubble, with the distant galaxies moving away faster. This can be supported by the Doppler red-shift of spectral lines of observed galaxies, the more distant galaxies being red-shifted more.