Dark Matter vs Dark Energy

Chris - We know that E = mc², Einstein's famous equation where energy and mass are interchangeable. Can the same be said for dark matter and dark energy? 

Colin - No. You're right to say that normal mass and energy are two sides of the same coin; You can convert one into the other. Whereas dark matter and dark energy appeared to be polar opposites; they're doing very different things. Dark matter is like a glue that helps stick things in the universe together, like a galaxy, for example. Whereas dark energy seems to be something that's doing the opposite. Not binding things together, but pushing things apart. Around the same time that Viagra came out, we discovered that the acceleration of the universe is accelerating. There's something between galaxies that seems to be pushing it apart. We call these things respectively dark matter and dark energy, but really they're placeholders for our ignorance. We don't know what either of those two things are. We just see the effects they seem to be having on the universe.

Chris - Yes, indeed. I did actually meet the guys that weighed the universe and worked out that things were not just getting bigger, but getting bigger, faster. We still don't know the mystery behind that. Did we? Because as space makes more space and distances become greater. It's almost as though it makes more dark energy. It's something of an enigma, isn't it?

Colin - This is another key difference between dark matter and dark energy. The majority verdict is that dark matter is a physical thing, it's a substance that exists in space. Whereas the dark energy seems to be a property of space itself. As space stretches, it becomes more and more dominant. It's another reason why the dark matter and dark energy seem to be different things. The only thing they have in common is that 'dark' word, and that's basically our ignorance.

Chris - One cynic said to me, that 'dark' is the word that physicists put in front of things to make them sound sexiest. So you get more grant money, but I'm sure that's not true.

Colin - The person who came up with the phrase 'dark matter', was Fritz Zwicky back in the 1930s. Knowing how eccentric he was, I can't imagine he was doing that. But dark energy? Yeah. Maybe they just thought 'good sounding thing'.