I understand that the redshift observed in other galaxies proves that the universe is expanding. However, I don't understand why that means that at one point in time the universe was infinitely dense. If a bomb explodes in mid air, all the parts of the bomb would be moving away from each other, but that does not mean that the bomb was infinitely small before the explosion. Why couldn't the universe have originated from something smaller than itself which still had a definite volume? This original "bomb" of the universe could have even come from some other, more complex entity which we simply cannot detect because it is too far away.