“We also don’t know why gravity is, but we know it is.”
How do we know gravity is?
We observe what happens to bodies (i.e. falling, why we can stand 'upside down" on earth and not 'fall" off, etc). We can also measure the force. It is detectable, consistent and it is universal. We just don't know why it exists. But you are free to doubt and jump off a cliff. That's why there are not that many doubters of gravity today—they're all at the bottom of some cliff.