No one has a definite answer. There might be a multiverse or a collection of “bubble” universes and our universe is only one of many.
Please also keep in mind that our human brain could be compared to that of a worm crawling on the surface of the Earth, totally unaware of the extra dimension containing the moon and stars. According to Einstein, if you board a spaceship and leave Earth on a straight course (never going left or right or up or down) and you think you’re heading for the end of the expanding universe it’s an illusion. You will always be sort of at the center of it. If you’re really going fast for a very long time you will get back to Earth. It’s like boarding a plane and always going straight.
People sometimes wonder what the size of our universe is and one answer is 93 billion light years. It sounds like a paradox given the limitation of the speed of light and the age of our universe which is approx. 13.7 billion years. There’s a rather complicated concept called co-moving distance. If you’d like to dig in deeper, just let me know.