To say that gravity is a phenomenon we know very little about is false to the fact.

We have been actively observing and measuring gravity since the seventeenth century. Between Newton and Einstein, we have detailed theories that are predictive to the limits of human ability to measure.

What you mean to say is that there are things that you would like to do with gravity that as far as we know cannot be done.

It might well be that these things just aren't possible. Not everything we can conceive of is possible.