1. The Arrow of Time. One of the fundamental puzzles of the universe is why Time moves the direction it does. Physics calculations work just as well backward as forward, and yet all the Time we see wherever we look seems to be moving the same direction as ours. (I'm not quite sure how the idea that Time could just as easily move back as ahead squares with the thermodynamic law that the entropy (disorder) of a closed system always increases, but I'll assume someone else already solved that and move on.)
In our universe, when I drop a rock, gravity draws it toward the Earth. It also draws the Earth toward the rock, just much much much much much much less. But if I run the film backward, to observers in our timeframe the rock and Earth seem to repel each other (I assume a native of that universe wouldn't notice anything strange at all). It's not really anti-gravity, it's just regular gravity going backward through time.
2. Dark Matter. As I mentioned a few posts ago, astronomers have figured out that the universe has much more mass than we can find. For example, if you add up the mass of all the stars in a galaxy then look at how that galaxy interacts with others, it acts a lot heavier than it looks. That missing stuff got the name "dark matter," though if I understand correctly it's better thought of as "transparent" or "invisible" matter; it's not like chunks of charcoal floating out there, but more like stuff that can't be seen or felt no matter how closely you look, refusing to interact with our regular ol' protons, neutrons, electrons and photons at all except through gravity.
At the same time, astronomers say that the universe seems to be expanding faster than it ought to. Galaxies and the very fabric of space between them are flying apart faster now than they did billions of years ago, even though common sense suggests they should be slowing down as gravity tries to pull everything together. It's almost as if there were some unknown repulsive force--some mysterious anti-gravity--pushing things apart. They call this "dark energy."
(My wife just walked in and, when I told her I was blogging about physics, she said "Ooooooh!" But I'm pretty sure that was sarcasm.)
3. My Hypothesis: Dark matter is nothing but a whole bunch of regular matter moving backward through time. What looks like a repulsive dark energy to us is ordinary gravitational attraction as seen by someone going the other direction. We can't see or touch the dark matter because it's playing by a different set of physical, chemical and electromagnetic rules, but we can feel the gravitational effects of its mass, the one characteristic that doesn't change no matter which direction time goes.
I leave the math as a trivial exercise for future grad students. QED.
We now conclude the wild-eyed crazyman portion of our blog. Have a nice day.
Me, earlier today
UPDATE 15 Minutes Later: Just uncovered a fatal flaw in my reasoning ("Only one?!" I hear you cry). Bad idea. Never mind. Still pretty sure my flux capacitor will work, however. All I need now is a DeLorean.