If the universe is expanding, then it stands to reason that in the past the universe was smaller than it is now. In the very distant past, it must have started as a very small entity, a super-dense point of pure energy, an energy singularity (often called the Cosmic Egg);  This universe was compressed to such an extent that temperatures and pressures must have been nearly infinite, making this singularity unstable. This unstability caused it to explode and expand. In the first fraction of an instant the universe was an energy-only universe, and as it expanded and cooled, all the matter that will form all the galaxies in the universe condensed from this radiation and has been expanding ever since. 

According to the Big Bang model, the universe began at one point at a specific moment in time. Because all the matter that exists in the universe was made at the beginning, its mass is essentially constant. However, because the universe is expanding, its volume is increasing and therefore, its density decreases over time. Moreover it is an evolving universe, whose appearance (singularity, big bang, energy-only universe, radiation and matter universe, etc.) changes with time. 

An alternate model, the Steady State model, conceives that the universe has always been expanding without beginning or end. Fundamental to the Steady State is the Perfect Cosmological Principle, namely that the appearance of the universe remains unchanged any time or any place that you look at it. Implicit in this statement is that, because the universe is expanding (its volume increasing) in order to look the same, its density has to remain constant. In turn, this means that new matter and galaxies must be forming (mass is increasing) in the gap that is created as the older galaxies move apart.