If the expansion rate is too small for the stuff within it, the Universe rapidly recollapses. If the expansion rate is too large for the stuff within it, the Universe rapidly dilutes so that no two particles will ever find one another.A universe with lower mass density would not form stars and planets. It's expansion would be so rapid that gravity would not be able to coalesce dust into stars.
Only if the Universe is “just right,” and I hope you’re saying “just right” the way you would when you tell the tale of Goldilocks and the Three Bears, can the Universe expand, cool, form complex entities, and persist with interesting structures within it for billions of years.
If our Universe, at the earliest stages of the hot Big Bang, were just a tiny bit denser or just a tiny bit less dense, or conversely expanded just a tiny bit more or less quickly, our own existence would have been a physical impossibility.
If the density of matter at the beginning of the universe's existence were the tiniest bit greater the increase in gravity would cause all stars to be much larger than the Sun. Such stars emit very high intensity radiation and are subject to rapid changes in temperature and luminosity, characteristics which would render any planets orbiting them unsuitable for life.
Slightly more mass in the early universe would've slowed the expansion such that all stars would become black holes and neutron stars. The density near the surface of such bodies would exceed five billion tons per teaspoonful. Atoms, molecules and life would all have been impossible.
If the density of the early universe were not almost exactly what it is we would not have either the right amounts or the right diversity of elements, and the expansion rate of the universe would've been either too great or too small.
So how finely calibrated must the mass density have been in the early stages of the universe's development to produce a life-permitting universe? Physicists have calculated the value to have been one part in 10^60.
This is a tolerance so incomprehensibly precise that according to astronomer Hugh Ross it's like an almost invisible fleck of paint on an aircraft carrier. If we compare this ship to the universe in its earliest moments, removing the fleck or adding one like it would make the carrier uninhabitable.
Or, to use a different analogy, the universe's mass had to be so precise at the beginning that if it deviated by the mass of a single dime it never would've been able to support life.
Moreover, as mentioned above, the rate at which the universe expands is also fine-tuned to a value of no less than 1 part in 10^90 and perhaps as high as 1 part in 10^120. To get an idea of the magnitude of just the lower of those two values, in the entire universe - 200 billion galaxies each with a 100 billion stars plus planets, dust, etc. - there are "only" 10^80 atoms.
The chances of a blindfolded person selecting by chance a specially-marked atom from anywhere in the universe are 10 billion times better than a universe just happening to chance upon the expansion rate necessary for living things to exist in it.
Imagine some sort of meter with a trillion, trillion, trillion, trillion, trillion, trillion, trillion calibrations. Turning a knob points a dial to precisely one of them, but if the dial was off by just one calibration the meter wouldn't work at all. That's roughly analogous to the fine-tuning of both the mass density and the expansion rate of the universe.
And these are just two of the dozens of finely-tuned constants, forces and parameters that must be set at these exquisitely precise values in order for life to be possible in our universe.
In the words of the the brilliant physicist Fred Hoyle, "A common sense interpretation of the facts suggests that a super intellect has monkeyed with physics, as well as with chemistry and biology, and that there are no blind forces worth speaking about in nature."
If that's so, the next question is what or who could that superintellect possibly be?
