Yes, yes, you can say "I think, therefore I am." But really? A fictional character in a book can say that, but it doesn't mean the character exists, other than as words on a page. And maybe not even that, if the book is in electronic form. In fact, a book in an electronic form is just a (big, complex) number. And numbers can exist even if there is no physical universe (you're welcome to check this by investigating the experimental case of no physical universe existing!)
But the same thing can be said of all of us. We (and all our cats and dogs and automobiles etc. - even the sun and the moon and the stars, if you wish) could be just a bunch of electronic files in a simulation. So our universe, and all its history and its future, is represented by a number.
It could be that there is nothing, no universe, nothing at all. But lots of numbers, and we are just parts of one of those numbers.