If you look at Einstein's equations and put in low speeds and low gravity, they become Newton's equations.

We account for all the matter and energy that we're familiar with, measure up how much gravity it should have, it's one-sixth of the gravity that's actually operating on the universe. We call that dark matter. It really should be called dark gravity. We don't know what that is.

One should not be too distracted by definitions. Ideas transcend definitions.

'As a fraction of your tax dollar today, what is the total cost of all spaceborne telescopes, planetary probes, the rovers on Mars, the International Space Station, the space shuttle, telescopes yet to orbit, and missions yet to fly?' Answer: one-half of one percent of each tax dollar. Half a penny. I'd prefer it were more: perhaps two cents on the dollar. Even during the storied Apollo era, peak NASA spending amounted to little more than four cents on the tax dollar.

No one is dumb who is curious.

Your center of mass is a place you cannot visit but you always carry with you. Like memories, it is part of life's baggage.

It turns out that the history of astrophysics is where we perfected time keeping.