The evidence of THINGS NOT SEEN. What we have is God's testimony to the finished work of Christ. That is enough, since Blessed are they that have NOT SEEN, and yet have believed (John 20:29).
The context is Christ's bodily resurrection. Blessed are those who have not seen the resurrected Christ and yet believe in His resurrection.
When the verse is applied to all areas of faith, it distorts the nature of faith. I've seen this in responses from other contributors, who think that observing or experiencing the miraculous is somehow contrary to or undermining of faith, or that those who experience the miraculous are "less blessed" than those who don't. Frankly, I'll take an actual physical healing over the nebulous "blessing" of believing "in faith" without the healing.
There's another nuance to this, one which I'd need to investigate in the Greek to understand better: "blessed are those who believe without seeing"... does that mean that they "will be blessed" (future state) or that they "have been blessed already"? I tend to go with the latter, because faith is a gift from God, so those who believe without seeing have been blessed with that ability/inclination to belief.
One more thing... the KJV has Hebrews 11:1 as "evidence of things not seen". I prefer some of the newer versions on this which use "conviction" in place of "evidence". It carries the sense of certainty, which "evidence" no longer carries (if it ever did).