There is no scientific consensus on what the universe is expanding into. Some theorize that the universe is expanding into a void or nothingness, while others believe that it is expanding into itself.

According to Big Bang theory - The prevailing scientific theory is that the Universe is not expanding into or onto "something," rather, the matter within the universe is what constitutes the Universe itself which, according to this prevailing theory originated via expansion from a microscopic singularity to form our current-day observable expanding Universe