Astronomers scouring the heavens with powerful telescopes can see objects that are billions of trillions of miles away. These observations have proven essential to piecing together a fairly refined picture of the history and evolution of the cosmos. Nevertheless, a gaping hole remains in our understanding of a basic question: What is the universe made of? For more than 100 years we’ve known about atoms, and over the past century or so we’ve gone further and identified atomic constituents like electrons and quarks, as well as their exotic cousins - neutrinos, muons, and the like. But there is now convincing evidence that these ingredients are a cosmic afterthought. Current data shows that if you weighed everything in existence, these familiar particles would amount to about 5 percent of the total. Most of the universe is composed of other stuff, which, with all of science’s deep insights, we’ve yet to identify.
- John Hodgman, Wired Magazine
This article also discusses other unknowns: What’s at Earth’s Core? Is time an illusion? How does a fertilized egg become a human? Etc. It's very interesting that despite the massive amount of scientific research and investigation that has occurred (starting with the Scientific Revolution in the 16th and 17th centuries, but particularly in the last 300 years) there is still a lot we don't know. Then, of course, there is the stuff we don't know that we don't know, the unknown unknowns.