There is no way to prove that the universe is open, flat, or infinite. These are all just theories, and there is no way to definitively test them. However, there is evidence that suggests that the universe may be flat.

One piece of evidence comes from the cosmic microwave background (CMB). The CMB is a faint afterglow of the Big Bang, and it can be used to measure the curvature of the universe. If the universe is flat, then the CMB should be uniform in all directions. However, if the universe is open or closed, then the CMB should be slightly distorted.

The CMB measurements suggest that the universe is very close to being flat. However, it is not exactly flat, so it is possible that the universe is open or closed. More measurements of the CMB and other cosmological data are needed to determine the true shape of the universe.

Even if the universe is flat, it is not necessarily infinite. A flat universe can be finite in size, but it would still have no boundaries. In other words, you could travel in a straight line forever and never reach the end of the universe.

The shape and size of the universe are some of the most fundamental questions in cosmology. We are still learning about the universe, and it is likely that we will not have definitive answers to these questions for many years to come.