Dark Matter: A Primer (PDF) — An excellent academic paper detailing the numerous pieces of evidence we've accumulated on the actual, measurable existence of dark matter.
Too dry? Here's an excellent Reddit comment (citing the above paper) that breaks down the evidence into more plain English.
Writing a program that writes itself — Quines are fun!
Want to know the easiest way to save time? Use
make is often a forgotten tool in the command line toolbox. But it's powerful, useful, and making a comeback.