IIRC there are two main pieces of evidence in favour of the Big Bang theory:

* Galactic red shift of galaxies
* Cosmic microwave background

The red shift is the observation that the light observed from distant galaxies is shifted towards the red (low energy) end of the spectrum. This effect is analogous to the Doppler shift observed with sound, e.g. if a car is driving away from you, the sound it makes is at a lower pitch than if it is driving towards you. The fact that the light is red-shifted is evidence that the galaxies are moving away from us at great speed. Specifically, the more distant a galaxy is, the faster it is moving away from us. The implication of this observation is that the galaxies are all moving outwards from some central point.

The cosmic microwave background refers to the observation that we observe a roughly constant level of microwave radiation everwhere in space; this observation is consistent with the prediction of residual radiation from the Big Bang; the amount by which this radiation has been red-shifted is the main piece of evidence used to estimate the age of the universe in the Big Bang theory (thought to be roughly 14 billion years).

As for whether the universe will continue to increase, I am no expert but I think this is not known for certain. The two important factors are the mass of the universe and its rate of expansion; for a heavy universe, the force of gravity will eventually overcome the expansion and it will collapse; for a light universe, it will continue expanding forever.

The rate of expansion is easy enough to measure, since it is related to the same red-shift observations I mentioned earlier. The hard bit is estimating the mass of the universe, especially since the bulk of the mass seems to be made up of so-called "dark matter" and "dark energy", which cannot be observed directly but whose presence must be inferred from the behaviour of nearby galaxies.


I admit I am not a cosmologist but I must say I was not aware of any known theory which explains these observations better than the Big Bang hypothesis. If anyone can suggest or link to one I would be interested to read it.

I am a little confused by those suggesting that the theory cannot be tested; the Big Bang theory makes predictions about observable quantities, which we can then look for. If we find them (e.g. the CMB) it is evidence in favour of the theory, if we do not, it disproves the theory. What other method of testing is there?