http://i63.tinypic.com/zjt0dl.jpg
http://i67.tinypic.com/302xczk.jpg
http://i68.tinypic.com/16au249.jpg
The organization of the screenshots is super messy, I compiled them over the course of several short breaks at work. There's a lot to unpack here because of how confusing the phrasing and terminology can be but I'll do my best in the limited amount of time I have right now.
"Darkness" almost always refers to the character and/or his card(s) with that in the name. "darkness" almost always refers to the 'gentle darkness', the force that opposes the "light of destruction".
"Darkness" embodies the "World of Darkness" which exists between the "darkness" and the "light".
"Universe" and "world" are often used interchangeably which causes problems when interpreting feats and statements. That said, when Darkness says universe, we can safely assume that he literally means that because his description of how 'the universe' was formed included the development of many other stars and planets. Unless we're to assume he meant "star system" but that seems highly unlikely.
The light of destruction is consistently referred to as a universal threat across multiple different arcs by multiple diff characters. This should put the light of destruction (which is an entity itself) at
3-A or
Low 2-C along with the gentle darkness and its wielder, the Supreme King. I believe it was also stated somewhere that the light of destruction has "reset" the universe multiple times before but I don't have screenshots of that so take it for what you will.
Now, for Yubel's feat. From what I understand, it was listed at
2-C a while back but was later downgraded to
5-B. I'd imagine that this was because of the conversation Judai has with the neospacian in his first encounter with them showing neospace orbiting Jupiter. We've never seen any kind of 3-D linear travel between the duel monster dimensions and the regular setting of GX at all. I know this probably sounds farfetched but we don't even know if that was the "same Jupiter" as should exist in the aforementioned regular setting. But we do know that Yubel, the one who was going to fuse and destroy said 'universes' referred to them directly as "dimensional universes". Merging them should at the very least be upper end
High 3-A (assuming the other 11 dm "dimensions" are not actually universe sized) or
2-C and destroying the combined universe could be either
3-A or
Low 2-C.
Darkness, himself, is pretty tricky. He embodies what appears to be a void that exists between light and darkness. Aaaaand welp, I'm out of time for now but I have an argument for
High 3-A Darkness with immeasurable speed and high godly regen coming so uh, yeah preview I guess