Is it possible to tear your acl or any ligament or tissue in your knee and never know?

Unlikely: Major ligaments about the knee require substantial force to tear. Sometimes though, injuries may not be fully appreciated for how forceful they were. Nevertheless, the patient usually remembers the injury. If no injury at all, then unlikely for any real ligament tears. If the knee has symptoms that include pain, locking, swelling and giving way, then formal evaluation should be performed.

Yes: Sometimes one will tear an ACL and not realize it, because the swelling from the initial injury subsides, they start getting range of motion and function back. Then they may try to "test" the knee, just to have it give out.