The fact that Rick and Morty live in just one of many universes opens up countless possibilities for exploration. From what we've seen, there are scores of different Ricks and Mortys out there (though many are Cronenberg'd monsters by now). And for every new reality, there are tons of tantalizing fan theories.
One of the most compelling of these explains an uncharacteristic plot hole in the series. Namely, that when we first meet Rick, he's been gone from his daughter's life for the last 20 years. We also know that Morty is 14 -- but later we see that Rick has memories of seeing Morty as a baby.
So either Rick snuck into see his grandkid in the night in an ill-advised nod to Superman Returns, or the Rick we know is from another reality. But why would Rick be hanging out with his family from another universe? Maybe most importantly, what happened to the Morty that Rick left behind? One theory suggests that we've already seen what happened to Rick's old Morty -- in fact, we see it every single time we watch the show.
Mixed in the montage of the opening credits there is a quick scene of Rick abandoning Morty to be devoured by large froglike monsters. Granted, we don't see Morty being eviscerated, but it would seem like that's a pretty rough spot. It might seem like just another goofy bit in the intro, but then again, lots of other scenes in the montage are ripped straight from the show. If this really happened and Rick's original Morty really did die, that would explain a lot about Rick's depression; part of the reason he keeps Morty safe (despite acting aloof the whole time) is to make up for losing the old Morty. Or maybe Rick is callous enough to know that he needs a new Morty to use the camoflauging ability of his grandson's Stupid Brainwaves.
It's a solid idea that makes sense with the world and its characters, but there's another explanation that's a little more complicated but a lot more tantalizing. Imagine that same scenario playing out as we see above, but instead of Morty dying, he survives and grows to resent Rick. Not only Rick, but all Ricks. What if Rick's former Morty is in fact Evil Morty?
Remember, in the episode "Close Rick-Counters of the Rick Kind," our Rick is brought into the interdimensional Council of Ricks to stand trial for murdering tons and tons of Ricks across the multiverse. I know, this show is weird, but stay with me. Whereas we originally think that the Most Evil Rick is responsible, the end of the episode reveals that Evil Morty was behind the scheme the entire time, controlling that Rick behind the scenes.
It's possible that, after being abandoned by "our" Rick, the original Morty held a grudge so vicious and unending that he decided that the only course of action was the wipe all Ricks from existence. It's also in line with the surpise we hear in "Evil Rick's" voice when our Rick tears up at the sight of Baby Morty.
Remember, this is actually Evil Morty talking through his puppet, Evil Rick. Though at first it sounds like condescencion coming from a fellow Rick, it's actually shock and disbelief coming from a Morty who thinks that all Ricks are incapable of feelings towards all Mortys. This is why we hear Evil Rick say "We both know that if there's one truth in the universe, it's that Ricks don't care about Mortys." It also explains why Evil Rick is so excited for a horde of Mortys to tear him apart. This is coming from a place of pure hatred. Some Rick out there has to have done something bad to Evil Morty to make him think that way.
It might have well been our Rick leaving him to die on a savage planet in the middle of nowhere. After all, it sounds like he's had run-ins with renegade Mortys before.
Could it be that Evil Morty is the instance that Rick is talking about? If Rick does indeed keep his promise to tell our Morty about it later on, hopefully it's in one of the handful of episodes left in season three.