They definately exist, in the form of boats and cars, you pour money in, and nothing ever comes out. And as absolute proof, I present the Visa card.

My thought is this, when humans dont have absolute proof they speculate, the objects that cause gravatational lensing and have effects on near by stars etc that we cant see certainly exist, until we can actually fly to one and observe it face to face we can only speculate, same as the big bang, it's a theory, for each advocate we can find someone who dispells it, I'm a fence sitter, it's an attractive theory that helps explain some phenomena we humans dont understand, my gut feeling is they probably don't exist and I say that without any scientific support.