Well into the 21st century, it is indisputable that we know more about the universe than ever before.
So that we don't get lulled into a false sense of confidence, today I provide a short list of open questions about the cosmos, focusing only on its composition. These are some of the mysteries that keep many fundamental physicists and astronomers busy and hopeful.
We know that 5 percent of the cosmos is filled with "normal" matter, that is, the stuff that we — and stars — are made of, mostly protons and electrons. We see the fraction of normal matter that shines or that reflects light. What we can't see in the visible we detect from the different kinds of radiation the matter emits in the whole spectrum of electromagnetic waves, from radio and microwaves to X-rays and gamma rays. There is a whole lot of interstellar gas and a lot of complex molecules spread around galaxies. Pretty much all of astronomy relies on collecting these different kinds of radiation — the sweat from the atomic dance.
The mystery comes from the rest of the stuff, the other 95 percent. There are two main characters — dark matter and dark energy. "Dark" refers to the fact that this stuff, whatever it is, does not radiate any kind of electromagnetic radiation. The dark stuff doesn't sweat, at least not in any ordinary radiating way.
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