I found this on comicvine, may help to clarify things
https://comicvine.gamespot.com/forums/battles-7/monarch-vs-white-lantern-kyle-1692043/
Credits to MaZeRaIII
Monarch did destroy universe.
During Superboy-Prime's entire fight with Monarch, taken From
Countdown to Final Crisis, is shown here:
imgur.com/a/4UA5A#1
First off, Scan 4 does include a panel in which Monarch managed to hurt Prime with a Quantum
"Nuke", but this Nuke had possessed a unknown yield - we don't know how strong the force of the Quantum blast was and, in fact, Panel 6 actually shows Monarch mention that his Quantum-based power is
"like the Big Bang itself". That being said, this so-called
"Nuke" would've obviously been leagues above any man-made Nuclear bomb.
In the same issue, we also see that Earth-51's Monitor, Nix Uotan, had still managed to survive the Earth-51 Universe's destruction. His personal shields didn't only save him and the surrounding radius, but also saved a Plant that had been nearby:
static.comicvine.com/uploads/original/10/102593/2801798-countdown13p21.jpg
Now, a lot of people try to debunk this feat due to the fact that not only did a minuscule part of it survived, but that there was some Light available to illuminate it, suggesting that the Sun was still there. This, however, shows a blatant ignorance of Astronomy as it takes 8.7 Minutes for Light to travel from the Sun to Earth - the Light in the panel had only been there due to being in the middle of travelling from the Sun to Earth, similar to how we can see long-dead Stars as if they were still lit up in Space. The Stars themselves are long dead, but the Light leaving those Stars actually takes thousands and thousands of years to reach Earth:
www.slate.com/blogs/bad_astronomy/2013/08/13/are_the_stars_you_see_in_the_sky_already_dead.html
Some of that Light was also likely from Monarch's Quantum explosion itself, but not necessarily all of it.
Despite the destruction, though, the Multiversal
"Orrery of Worlds"survived the destruction of Earth-51, and repairs were immediately started, as was shown and stated in
Final Crisis #1:
images.sequart.org/images/Orrery.jpg
"The Multiversal Orrery has survived repair after the loss of moving part: Universe 51"
Solomon, the Monitor of Earth-8, actually confirms In
Countdown to Final Crisis that the Earth-51 Universe had been completely destroyed and
"reconstituted" afterwards by the Monitors - it makes reference to the
"Morticoccus", as well, an adaptive strain of the OMAC Virus that ravaged the newly-reconstituted Earth-51 Universe
after it was destroyed and repaired from Monarch's explosion:
static.comicvine.com/uploads/original/8/89194/3044871-countdown4p13.jpg
"Even though I've wrested the great disaster from him...allowing the Morticoccus to thrive in Nix Uotan's reconstituted Fifty-First Universe instead of that original Universe--essentially destroying the same Monitor's domain twice..."
dc.wikia.com/wiki/Morticoccus_Virus
A panel taken from
Final Crisis #7 elaborates further on this point, fully describing how Earth-51 had been repaired by taking pieces of other Universes (Moons, Planets, Stars, ect...) and adding them to the Earth-51 Universe - this had been just after Darkseid died in
Final Crisis:
i.imgur.com/uwgjcky.jpg
"I saw the World remade with my own eyes. With pieces from other times, other places. All in a vision that came to me in Command-D"
That being said, after Earth-51 was repaired and then ravaged again by the Morticoccus, the entire Universe had been barren by the time that Superman and the Ultraman of the Anti-Matter Universe arrived there in
"Final Crisis: Superman Beyond #1":
static.comicvine.com/uploads/original/6/60791/3015699-1216813080-42154.jpg
