It's a mistake to say all flat-earthers believe any particular explanation. The bit about "dark energy" is something I've never seen in the year or two I've been reading flat earth social media. One of the more common arguments I see from them is that humans should be able to feel the earth spinning and that the oceans should be flung off the globe, so claiming that earth is accelerating upwards would be problematic.

Not that their ideas are always internally consistent.