The universe is actually expanding and much faster than we originally thought it would! It’s not necessarily that more matter is being added in but that the mysterious force of dark energy is overpowering gravity and creating more and more space. We think this space is empty and that its expansion is just a natural property but some scientists speculate that it might not be empty at all and is instead filled with particles popping in and out of existence and the energy the particles leave behind is what makes up the force of dark energy. Normal matter (you, me, and every galaxy) makes up just 5 % of the universe so it’s not to blame for the expansion we see.
At one point we thought gravity would bring the universe’s expansion to a halt or even reverse it but it’s curious to see quite the contrary is happening. Unfortunately we won’t know anything for sure until we unlock the mystery of dark energy and dark matter.
Thanks for reading, I always look forward to hearing from you.