It can't be an illusion. Although some universal aspects are abstract and an illusion is an abstract feel, the universe does exist, in its vast, infinite shape, it's true, but it is there. Use the Argumentum ad absurdum principle: If the universe wouldn't be real, then you wouldn't be real either, because you are part of this infinite set, more exactly, the subset of living forms. But you think, you are governed by ration and you use your senses to explore the world, a physical world, described by scientific laws. Therefore you ARE real; but wait, this is a contradiction, which means that the hypothesis is false, thus, the universe exists.
This is the simplest form the argument can be stated. Of course that the problem is more complicated than this, because you can think more abstractly and ask questions about space and time dimensions, living forms / conditions and equations of the universe.