There is no singular "cause" of matter. Matter is a fundamental aspect of the universe and does not have a cause in the same way that events or phenomena do. Matter is composed of fundamental particles such as protons, neutrons, and electrons, and these particles are believed to have come into existence during the Big Bang, which is the prevailing scientific theory for the origin of the universe. The Big Bang is thought to have been a rapid expansion of space-time, during which the universe rapidly cooled, allowing for the formation of subatomic particles and eventually matter as we know it. So, matter is a product of the fundamental processes and interactions that occurred during the early moments of the universe.