Just some basic questions about gravity. What is gravity, really? What causes it, just the existence of mass alone? <br /><br />What are the knowns vs. unknowns about gravity, and how it works?<br /><br />I have also seen some theories about the "space-time continuum" on astronomy shows, and about how gravity creates something similar to a "depression" in it. How much is really known about this elusive "membrane" of existence?