An absence of proof isn’t proof of absence. I have no idea what the Mueller investigation is finding, has found, and will reveal. Can they show Trump colluded? We may never know. What I can say is that if Mueller and team show no proof then Trump will tout it as vindication that nothing happened. All it really would mean is that Mueller didn’t find evidence.
As a case of illustration, I’m connected to a friend on Facebook. I think the last time we saw each other was in May 1986 when we moved out of a house we shared. I could claim that I don’t know him, but it can be proven otherwise. There may still be a copy of a lease somewhere. We had roommates, others knew us. Proof exists to refute any denial I might make. Conversely, I can say that I’ve never met Barack Obama. I haven’t, but I can’t prove it. Yeah, it can be proven that I never visited the White House while he was president. They have visitor logs, background checks, and other ways to show that I haven’t visited. But I can’t, no one can, prove I’ve never met him. After all, perhaps our paths crossed on a Cambridge street when he was at Harvard.
An absence of proof isn’t proof of absence.