1. Galaxies are moving further apart and the red shift of distant galaxies leads us to conclude that they were once closer together before some kind of expansion which proves that the universe must be expanding. Using the Doppler effect to notice the change in pitch of sound depending on whether it is approaching or moving, it is formulated that the universe is expanding as the movement of birth stars and galaxies away from the Earth is measured.
2. The Big Bang Theory would explain the abundance of elements in the universe including hydrogen and helium. The singular way that helium can be produced is by the nuclear fusion reaction that takes place in stars however there is far more than could be produced by the stars alone except the percentage of helium atoms can be explained by their synthesis resulting from the big bang.
3. George Gamow predicted that due to the temperature of the universe, it would be emitting an 'afterglow' of radiation as anything with a temperature above absolute zero emits radiation. 'Cosmic microwave background radiation' was coined as a term for the afterglow which is part of the Electromagnetic Spectrum. The radiation was discovered as a consistent radio noise that wasn't coming from Earth, but rather from all directions out on space, and the Big Bang theory is the only supported theory to explain this radiation.