What does the universe expanding mean?
Around 1920 Edwin Hubble studies the spectrum of Galaxies and found that they were showing red shift.
That means they were going away and the more they are far away the more speed they were hiving..Now it is known that this is due to the big bang which happened about 13.82 billion years back Universe is expanding and the space between Galaxies are increasing..But local gravity keeps local systems free from expanding.So solar system is not expanding.
Recent studies shows that the universe is not only expanding but accelerating and reason may be Dark energy.
the increasing rate of the expansion of the universe means that our present observable universe is not eternal. nor self existent.
Before Hubble's observations of the red shift in 1920 almost all scientists believed in the steady State theory of the universe. The red shift lead to Big Bang Theory and the realization that the universe was not static but changing. The red shift bought the realization that the present universe had a beginning.
Before 1990 and the observations of supernovas giving empirical evidence that the rate of expansion of the universe was increasing, most scientists accepted theory of an oscillating universe. The idea that matter and energy were eternally oscillating between a period of expansion ( Big Bangs) and periods of contractions ( Big Crush)
The evidence of an increasing rate of expansion brought the realization that the present universe will have an ending.
The expanding universe means that matter and energy are not eternal nor self existent. The present observable universe had a beginning and the present observable universe will have an ending. This means that the universe is an open system and something besides the the observable universe must exist. This creates a major problem for the philosophy of material realism that matter and energy are all that can exist.