There is a limited number of ideas that hold up under examination, however insistent, repeated, or challenging. One of these, so far, is "Everything in the universe attracts everything else with a constant and unfailing force". We know it here as gravity. In our solar system, the planets zoom about in elliptical orbits that slowly decay over time. Eventually, the sun will win and swallow the planets. Further out, all the stars attract all the others, affecting the motions of stars within groups of stars (galaxies), within groups of groups of stars (galaxy clusters). The attractive nature of stuff, or matter, cannot be denied.
Since stuff accounts for the motion of stuff in a well-behaved way, astronomers can use mathematical formulas (ranging from simple to elaborate) to map these motions, and to determine where all these objects are located. Of course, as a first approximation we can begin with the naked eye, as humans have done for millennia. We can see an impressive amount on a clear, moonless night. And we are quite sure that most, maybe all, of the stars we see have more stuff orbiting around them, ranging from dust to planets. But what about the rest of space, where there are no visible objects of which we are aware? Apparently something in "empty" space also affects the motions of celestial bodies -- something called dark matter. It may be spread so evenly, or have such elusive properties, that it can never be found by our very sophisticated instruments. However, indirect evidence says it exists nonetheless, this elusive matter, this condensed energy.
So there you have it -- stuff, whether visible or invisible, attracts. This principle is as commonplace as a falling apple, and as enigmatic as dark matter. Yet lest you attribute too much power to this wonderful force called gravity, keep in mind Einstein's wry disclaimer: "Gravity cannot be held responsible for people falling in love".