Black Holes.

A black hole is an object so dense that its gravitational field is so intense that nothing, even light, can escape its surface.

The size of a black hole, at least one that's not spinning around (rotating), is given by:

Black holes are quite dense. Listed below are the sizes of black holes (or more accurately,
their Schwartzchild radii) for some well known objects:

Object NameMass (kg)Schwartzchild radius
Earth5.9724 x 10240.890911 cm
Sun1.9891 x 10302.953518 km
Deneb1.2 x 1032178.182183 km
Our galaxy5 x 10414982.723247 AU
Virgo Cluster104415.694944 light years
The Universe1053 1.57 x 1010light years

The mass of the universe is based on there being about 5 x 1011 galaxies (Based on the Hubble deep field count) and that each galaxy is about half the mass of the our galaxy, the milky way.
The last number, 1.57 x 1010light years is quite interesting. It's well within the error bars for the current observed radius of the universe.

Our Universe is a black hole!!!

Why this is so is unknown. It might be just a coincidence, or it could have deep physical meaning. No one knows.

The background image used is of the open cluster M67. It comes from the Palomar sky survey I.