I actually think they already told us what happened. Very sneakily, clues have been dropped throughout the show.
We know that <PERSON> was unhappy with married life-
"Listen <PERSON>, I hate to break it to you, but what people calls "love" is just a chemical reaction that compels animals to breed. It hits hard, <PERSON>, then it slowly fades, leaving you stranded in a failing marriage. I did it. Your parents are gonna do it. Break the cycle, <PERSON>. Rise above. Focus on science."
But we also know that <PERSON> had an unconventional way of raising beth, from what <PERSON> has said during one of his arguments with beth(Mentioning that she was "Raised like a lab rat"), which leads me to believe, from <PERSON>'s nonchalance with most of <PERSON>'s work and apparent knowledge about how some of it works, that she may have been the "Original <PERSON>", or <PERSON>'s original sidekick.
<PERSON> also stated during the season 2 finale of <PERSON> and <PERSON> in the space van that they could never return home now that they're galactic federation terrorists, and in addition to that, within <PERSON>'s microverse battery, <PERSON> revealed he dropped out of high school, and according to the show creators, <PERSON> is supposed to be a sort of doppelganger to <PERSON>.
However, <PERSON> may have actually revealed <PERSON>'s method of leaving. In the episode Auto-erotic assimilation, when <PERSON> seemed nonchalant about the concept of not having underground secret labs, <PERSON> says these words:
"Really? Are you sure you're not gonna, like, say okay then leave through a portal or something?"
It should also be mentioned, another detail, that it's been stated in the show that once you become a galactic federation enemy, you can't go back to earth. It might be the case that <PERSON> didn't leave just because he was an amoral alcoholic, but because he literally couldn't, even if that's really what he wanted. However, one has to wonder if that's the case, why <PERSON> didn't just tell <PERSON> that.
There's another aspect to be considered, and that aspect is revealed in the comics. In the story of <PERSON>, <PERSON> starts doing business with <PERSON> during his time as a member of an alien band, or "The flesh curtains". 663
But that takes place before <PERSON> claims that he thinks he's in love. In other words, it sounds like he began his science work before he had gotten married and had beth.
From those things, I've pieced together a theory/headcannon:
<PERSON> was a genius that dropped out of high school and became a mad scientist. At some point, he acheives his main mode of space transportation(Which may not have necessarily been his current space cruiser, since it's possible the night of the pilot episode was the night it was made), and meets <PERSON> and birdperson(And eventually, they start a band). Since I doubt he got a job, let's assume "<PERSON>" is carrying most of the weight financially. <PERSON> meets <PERSON> and have <PERSON>, and <PERSON> works as a home mad scientist, possibly using his inventions to help the family and possibly taking beth on adventures the way he does with <PERSON>. I think he probably made his space cruiser first, and with his space cruiser he could have gotten the resources to make his portal gun. Whether he got help from another <PERSON> like in the "Fabricated" origin story or he discovered it himself, he eventually achieves inter-dimensional travel. He's already smart, but it's possible he somehow got his hands on mega seeds and made himself even smarter.
It's also possible <PERSON> and <PERSON> did die, and he swapped dimensions as he did in the episode "Rick Potion #9", which has to lead many fans to believe he may have done it before. In any case, it seems that married life probably bored him, since we know to rick, if it's simple, it's boring, and the nature of an infinite multiverse in which he concludes "Nothing matters" probably drove him into nihilistic depression. It's uncertain when exactly he becomes a galactic federation enemy, but at some point he does, and it can be assumed <PERSON> and birdperson join him in this. I believe the galactic federation likely became his enemy for 2 reasons. The first reason is a mutual disagreement on each other's views about society and bureaucracy. 663