But the most astounding fact about the universe to me gets eloquently explained by Neil deGrasse Tyson in this video:
It is rather impressive to think that everything you see around us is made from atoms forged in the blazing hot cores of exploding stars. In its dying moments, a massive star explodes with unimaginable force and is called a supernova. The immense temperatures and pressure at the star's core result in runaway nuclear fusion processes that produce lots of heavy elements. These are then ejected into space by the explosion and eventually, many, many years later cool down and form into new stars, planets, asteroids and pretty much everything else you find in a typical solar system such as ours. Quite amazing.
A supernova can lead to other, deeply fascinating things like neutron stars and black holes. Neutron stars are the extremely dense remnants of supernovas that were not heavy enough to fully collapse into a black hole. They are made entirely of neutrons and can in some way be seen as a massive atomic nucleus. They manage to squeeze about 1.2-2 times the mass of our sun into a region about 12 kilometers in diameter. A single teaspoon of neutron star material weighs about 900 times the Great Pyramid!
Then there are black holes, the fabled and ultimately mysterious objects that lurk in the universe. Infinitely dense and small regions of spacetime where physics (and reality) as we know it breaks down. Nothing can escape from them, in some ways the ultimate testament to the indifference of nature. There is a lot more that can be said about black holes, maybe in another post sometime...