QAHow do we know that the universe didn’t always exist?
According to the second law of thermodynamics, the amount of usable energy in the universe is decreasing. The universe does not have an infinite quantity of energy. Since everything that is finite must have a beginning, the universe did not always exist.
The premise that the universe had a beginning receives strong support from natural science. Science has shown that the universe is steadily expanding: therefore, at some point in the distant past, it came into being. The Big Bang theory, supported by the discovery of a radiation echo in the universe, points out that the universe is not eternal, but had a beginning from a single source.
Finally, we observe that all matter changes and is not eternal. Therefore the universe cannot be eternal.