Since 2017, and first military footage leaks there is proof. That's why all this is such a big deal. Maybe it hasn't been explained well, but the pentagon has confirmed the footage is real. The videos are from military aircraft, and equipment. We have pilots who have come forward confirming the encounters we see in the videos. The Pentagon says the crafts are not theirs.
I've said before in this thread we only have 3 possible answers:
1. The Pentagon is lying, and this is some secret tech.
2. It's a foreign power's secret tech.
3. It's aliens.