Joel Achenbach in the WaPost

The field of cosmology is constantly changing.

Stephen Hawking’s best-selling 1988 book, “A Brief History of Time,” posed big questions that remain unanswered 36 years later: “Where did the universe come from, and where is it going? Did the universe have a beginning, and if so, what happened before then? What is the nature of time? Will it ever come to an end?”

Here’s another stumper: No one knows what the universe — including “empty space” — is made of at the most fundamental level. The physicist Brian Greene, in an email, framed that question like this: “What threads stitch the fabric of space?”