You mean (sorry they propose) that the 20% of universe's mass is composed of those right-handed neutrinos?

Yes, this is exactly what the model predicts.

Because as you mentioned they are not observed yet. And maybe impossible to detect. Which makes them only theoretical particles such as axions or WIMPs. And by the way, Has this theory any explanation about dark energy as well? (I've taken a quick look at the paper you've linked, however, I need to read it at a later time in detail)

They are by no means axions or wimps because they just don't interact at all with the Standard Model. They are not thermal relics but dark matter is here produced via another mechanism connected to gravity.

PS: sorry for the disappointment. There is a second link a couple of sentences later in the post :)