for anyone interested in the lore behind the gates, their shutdown and the following chaos, i recommend reading
this old old blog post. i believe a few details may have been changed since it was written, but it still explains the general idea of why the Sector is what it is.
TL;DR: the Sector (= everything we see and can travel to in-game) was once merely a fledgling colony of a massive empire (the "Domain of Man"), with gates allowing instantaneous travel between the empire's various regions. about 200 "cycles" (aka years) ago, all the gates in the Sector suddenly closed without warning or explanation.
since the rather recently colonized Sector was still dependent on the Domain for supplies and technology necessary for survival and further terraforming, the local economy and order quickly collapsed, as people got desperate to secure their own survival. even two centuries later, much of the Sector is still plagued by poverty, crime and war, and available technology has been slowly but surely deteriorating.
like most major factions, the Hegemony was formed
after the Collapse, as a self-appointed successor to the law and might of the Domain. iirc, Tri-Tachyon existed as a megacorporation before the Collapse, but what we know as Tri-Tachyon in-game was formed from just the local branch of the original company that got isolated when the gates closed, and has since become a ruling faction just as much as a business corporation in order to ensure profits in the lawless environment of the Sector.
i don't think we will ever get an answer on why the gates closed, as that mystery is kinda key to Starsector's background story. and i don't think we'll ever reopen them, since the game focuses on
the Sector and its struggle, which would be kinda moot if the Domain returned to save us all. if the gates do reopen at some point, i imagine it would be to create a new endgame danger through some kind of massive invasion force that threatens the entire Sector.