According to Einstein, massive objects have immense gravity that actually distort space and time. If you consider our Earth, which is a massive object in space, its gravity is not a "force that acts to pull on objects" but rather, it actually warps the surrounding space and time. The moon actually orbits the earth because it is following the curvature of distorted space around the Earth created by the Earth's warping of space. In the great video (Top by Kurdistan Planetarium) there is a good explanation of how a massive object (Sun) warps space causing the Earth to orbit it.
In the middle image above (from Quora.com), the curvature of space-time created by the Earth is represented by the blue lines creating sort of a "distorted web".
Now consider, a larger object such as our Sun which is thousands of times larger than our Earth. The Sun's gravity is thousands of times larger than the Earth's so its radius of influence is larger, which means the distance that it warps space and time is thousands of times larger than the Earth's. That is why so many planets orbit the Sun.
Image compliments of Quora.com
Now imagine the extreme influence and warping of space created by a black hole, which distorts space so much that it is orbited by an entire galaxy thousands of light-years in diameter. Such is the power and strength of gravity exerted by a super massive black hole at the center of each galaxy!
In summary, here is a diagram (below) that depicts the progressive warping of space caused by progressively larger objects that have progressively larger gravity.
The ultimate object, is of course a singularity or black hole, the largest of which, are supper massive black holes at the center of every galaxy.
How cool is that.
Maddalena Environmental Inc.