"Often considered" by whom? Certainly not by respected historians.
Ironically this was the period in which Christianity had the strongest hold over Europe (for comparison, the Islamic world, China, Mesoamerica, and more regions of the world were thriving, with various problems but they were still better off than Christian Europe).
This Mesoamerica?. Come on now. Admit that was a bad example.
As the Enlightenment arose and more secular ideas prevailed, the world gradually got better in almost every way: science and technology flourished, morals continued to evolve, and while violence and crime continued at incredibly high rates, it was less common.
Science and technology were already making huge advances since the Renaissance. You all might have heard of this person.. Da Vinci, for example.
I'm not sure /r/AskHistorians is really the place to adjudicate reddit debate subs.
Certainly, the notion of the 'dark ages' has not been seriously used by respected historians (at least those specialising on relevant subjects) for over 100 years. And the broader notion of a "dark" middle ages was already on its way out by the mid-19th century as part of the broader romantic, nationalist appropriation of the middle ages. I have written about this in part here. See also this thread, and particularly /u/sunagainstgold's comment, on the most recent popular reincarnation of the "dark age" trope.
But at the same time, equally few respected historians would describe the middle ages as "light compared to all of history". This sort of claim, or at least a certain positive nostalgia for the Middle Ages, certainly crops up in some of the work of particularly the more apologetically driven, popular Catholic historians. But the closest we get in respected academic writings is the sort of nostalgia found in, for example, Brad Gregory's The Unintended Reformation, wherein he writes positively about the widespread lay piety in the late Middle Ages and the, in his eyes, well founded philosophical-theology of the era, and then goes on to describe how both of these were in one way or another lost through the extended process of the Reformation. And while, I think not entirely unfairly, a number of reviews have suggested that he is promoting some sort of pre-modern nostalgia, Gregory strongly opposes this sort of characterisation, even pre-empting this charge in the title of his conclusion: "Against Nostalgia".
So you're certainly not on any firmer ground trying to claim that the Middle Ages is in any relevant way brighter or better period than that which came before or after.
As to your interlocutors claims about the enlightenment, they are essentially taking Pinker's line, which has not been generally well received by historians. Not so much because the notion of a reduction of violence into the modern world is inherently implausible, but because even granting his dodgy interpretation and application of historical statistics, Pinker does no serious work interrogating or contextualising this trend. It serves his simple, wiggish and self-congratulatory account of the ascendency of the "The West". On this point, I think /u/tiako's brief comment here is extremely sensible and really cuts to the heart of the problem. And of course, his point about the enlightenment is perennially ironic given that he doesn't seem to have ever seriously read what actually enlightenment thinkers wrote.