>Hi Hilmar,
>
>Could it be that the matter or mass of the universe remain constant, but the dimensions of the universe vary?

Making some calculations with the General Theory of Relativity, the Universe can't remain constant, but would, in the long term, either expand or contract. Einstein didn't like this idea, so he invented a "cosmological constant" to counteract these forces - an idea he later rejected.

(The "cosmological constant" has recently attained renewed credibility, however, due to the observations, that indicate that the expansion of the Universe is accelerating. This cosmological constant would be like some sort of force that acts on a large scale.)

It was later observed that distant galaxies change their spectrum (observed light) towards the red end - meaning that they are apparently receding from us.

It is commonly believed, by most present-day astronomers, that the Universe started something like 14,000 million years ago, in a "Big Bang", in which matter was extremely concentrated.