[jmyeet]

The most likely alternative energy soruce is fusion. Personally I'm not convinced this will ever be commercial. I'm not saying it won't be but there are huge problems to overcome, most notably destroying the container and plasma turbulence.

We already have the tech for space-based solar. Putting it in space makes it much more efficient. It's relatively low tech and doesn't require exotic materials or solving the massive engineering problems that fusion (for example) does.

Then there are other way more exotic methods (eg antimatter, black holes).

But all this misses the point. If you generated 10^26 Watts of power on Earth using fusion power you would still need to dissipate that heat into space or you'd quickly cook the Earth. So you're pretty much back to where you started.

What I like about the Dyson Swarm idea is that it requires no new physics and is relatively low-tech. So even if there is something far-future that's an option (eg antimatter, black holes) you have to ask questions like: Will everyone reach that point? Will civilizations build Dyson Swarms as an intermediate step anyway? How long is that reality? Will there still be a mixed-use future with both space-based solar and exotic enegy generation?

The Sun radiates a ton of energy into space. Capturing it is relatively simple. Dismissing that seems to defy credulity (IMHO).