Is there a void at the center of the universe?

The universe does not have a center. It does not have any boundaries. There’s no reason for it to have an empty space. It started as a relatively small homogeneous concentration of energy and expanded uniformly.

What if the universe is collapsing?

Astronomers once thought the universe could collapse in a Big Crunch. Now most agree it will end with a Big Freeze. If the expanding universe could not combat the collective inward pull of gravity, it would die in a Big Crunch, like the Big Bang played in reverse.

Does the universe lose matter as it expands?

So yes, Andrej, matter particles are affected by the expansion of the Universe: they cool and lose energy. It took not only the radiation to drop in energy, but also the individual energies of the particles in the Universe to drop in kinetic energy to form the molecules, stars, galaxies and planets we see today.

Is it possible that the universe is static?

Yes it is possible that the universe is static. In your imigination line up a row of nails end to end on a hot summer day. Line them up for a mile end to end. Mark the beginning and the end of the row.

What are the characteristics of Einstein’s static universe?

Einstein’s static universe is closed (i.e. has hyperspherical topology and positive spatial curvature), and contains uniform dust and a positive cosmological constant with value precisely , where is Newtonian gravitational constant, is the energy density of the matter in the universe and is the speed of light.

What would happen if the universe was filled evenly with matter?

If all you had was a Universe filled uniformly and evenly with matter, that matter would simply get less dense and would see everything expand away from everything else as time went on.

Why is the universe so unstable?

It is unstable in the sense that any slight change in either the value of the cosmological constant, the matter density, or the spatial curvature will result in a universe that either expands and accelerates forever or re-collapses to a singularity.