Everything certainly could be illusion.
This is the stuff of Plato's cave and Descartes' musings and "The Matrix" and late night dorm room discussions.
Been there, done that.
And then it's time to get back to real life.
Scorpion, try this experiment:
Pretend you have an imaginary pin in your hand and stick yourself with it.
Then, do the same with a real pin.
That, on a fundamental level is the difference between illusion and reality.
You can ponder that the real pin is just an illusion, BUT IT STILL HURTS!!!
And so most sane people, after questioning if everything is an illusion, must run their real live "as if" matter exists and cause and effect are related.
To do otherwise can put one's existence in this plane of reality in jeopardy.
But good luck finding other planes and assuming matter is illusory. Just don't hurt yourself!