Actually Eternity first appeared 4 months before Galactus did. Eternity debuted in Strange Tales 138 cover dated Nov 1965, Galactus is first mentioned in FF 48 cover dated March 1966 and Tribunal was shortly afterwards in Strange Tales 157 in June of 1967, so none of them were the be all and end all to be outdone later int he MU, they all kind of came out around the same time when books like FF and the Doc Strange strip in Strange Tales were pushing the limits with Kirby ad Ditko at the helms as they were "exploring the nature of the universe" (as you put it) and just telling stories.
As for exploring DC's cosmic side, it depends if you want contemporary stuff or older stuff.
If you are looking at older stuff-
The 80s Omega Men series
Cosmic Odyssey by Starlin & Mignola
Hardcore Station by Starlin
Adam Strange: Planet Heist
Kirby's Fourth World Saga
a few short lived 70s series too
Star Hunters
Starfire (not the Teen Titans character but the sword & science heroine)
If you are ok with tracking down back issues/reprints, a good sampler of DC sci-fi characters are these issues of DC Super Stars...
you can also check out things like the Byrne/Mignola World of Krypton series (that is serving as inspiration for some of the current Krypton series on SyFy)
If you are looking for cosmic entities playing a role in the stories, you can check out the Doctor Fate stories when he was a back up in the Flash (collected int he Immortal Dr. Fate mini) and the later Keith Giffen mini-series and the J.M. DeMatteis penned onoing formt he late 8ps that feature the Lords of Order and Chaos in prominent roles. It's magical cosmic much int he way classic Dr. Strange was a cosmic series for Marvel rahter than cosmic in the typical sci-fi mold.
If you are looking for more contemporary stuff. others have mentioned the GL stuff and the upcoming No Justice series, but Mister Miracle gives you a glimpse of the Fourth World stuff, and the recent Omega Men (well now a couple years old) can give you a glimpse of some fo the stuff.
-M
Comic fans get the comics their buying habits deserve.
"Opinion is the lowest form of human knowledge. It requires no accountability, no understanding." -Plato
The term cosmic can have many meanings. jm dematteis, for example, called characters like Pandora, Phantom Stranger, and the Question cosmic beings.
In terms of heroes, DC has planty: LOSH, Green Lantern mythos, Omega Men, LEGION, Captain Comet, New Gods, etc...
All with huge history.
Swamp Thing goes cosmic...
-M
Comic fans get the comics their buying habits deserve.
"Opinion is the lowest form of human knowledge. It requires no accountability, no understanding." -Plato
GDS isn't the start of Levitz's run. He had already written the book years before and was already into his second stint when he wrote the Great Darkness Saga.
It is a pretty cool way to start reading LOSH, thought. I don't think there's ever the perfect spot to start reading LOSH's first continuity. So many characters and plotlines at once, that you simply got to pick and issue and go from there. Unless you really want to start at the beginning or go the reboot route.
I was always interested in the Source Wall but had no idea until recently that it first appeared in the very Titans vs XMen story I first saw it in.
I resurrect this thread because of the Empure event going on at Marvel.
As you may know already, Justice League Odyssey is coming to an end in october, and that plus Empyre makes me wonder if DC isn't wasting away its cosmic (galactic perhaps) corner. I mean, is there even one race/empire as developped as the Kree/Skrulls are at Marvel (and I know there is also the Shi'ar) ?
Abnett started to do it a little in Odyssey with the Tamaranean troops (who looks sublime) but I have the feeling the galatic corner of DC will be fully back to Green Lanterns soon, which is a damn shame...