The more matter you squeeze into any finite area of the space-time continuum, the stronger are the observable effects of gravity. We are comprised of matter, and so the collective weight of all our atoms further bends the space-time continuum.
We are as much affected by gravity, as we are the cause of it. In fact, it is not simply the earth which exhibits gravitational force on us pulling us towards it, but the gravity of our own being pulls the earth towards us.
When enough matter squeezes into a small enough area a black hole is formed, a gravitational phenomena so strong not even energy in the form of light can escape it's gravitational force.
From inside, a black hole might appear to be brighter then the brightest star (our sun is a star), but because no light escapes it, we cannot see it. All we can see is a disc of matter spinning around it, caught in it's gravity, falling into it.
