The Mirror Universe is a separate galaxy expansion where the Terran Empire rules instead of the Federation -- around 50 new systems, reachable only from Ops 40 and only with a ship carrying at least 50 warp range. It's not somewhere you wander into by accident: getting in, staying in, and getting back out all run on their own rules, and treating it like normal space is how you lose a fleet's worth of cargo in one bad warp. The NSEA Protector is the ship built specifically around operating here -- see the guide linked below if you're gearing up for a serious push.
Entry runs through Interphasic Entry Rifts, found at specific warp locations -- Mirror Alpha Centauri, Mirror Wolf, and Mirror Sirius are the known entry points. Each ship that warps in costs 300 Mirror Universe Entry Vouchers, and that cost is per ship, not per trip -- bringing three ships in costs 900 Vouchers, not 300. You can hold up to 900 Vouchers claimable from the Mirror Refinery every 3 days, with more purchasable at the Store if you need to move faster than that regen rate allows.
A Voucher buys you 20 minutes of base duration inside the Mirror Universe. Run out the clock and your ships auto-extract, keeping whatever you'd gathered up to that point -- so the timer running out isn't a punishment, it's a safety net. Leaving early works differently: you can't simply recall a ship out of Mirror Space the way you would anywhere else. Instead you need to physically find an Extract Rift and use it, and Extract Rifts never cost anything to use. The one way to leave badly is to get destroyed before you reach one.
Moving between Mirror Space systems needs a Rift Key, and the rarity you need -- Common, Uncommon, or Premium -- depends on how deep into the Universe you're trying to travel. Rift Keys come from the Mirror Refinery, the same building that supplies your Vouchers. None of the game's usual instant-travel tricks work here: Discovery's Summoning, Borg Cube warping, Gorn wormholes, and similar shortcuts are all disabled inside Mirror Space, so plan routes the way you would have before those abilities existed.
Trellium only exists in Mirror Space, in two raw variants, A and D, obtainable either by mining dedicated Trellium nodes or by destroying Terran hostiles. Raw Trellium has no use on its own -- you have to refine it into Liquid Trellium via the Mirror Universe Refinery, which unlocks at Ops 40 alongside the rest of Mirror Universe access. Liquid Trellium-D specifically is required to upgrade officers who originate from the Mirror Universe, like Picard and Data at launch, and it's also spent directly in the Store.
This is the detail that catches people out: there is no protected cargo inside Mirror Space. If a hostile or another player destroys your ship while you're holding raw or refined Trellium, you lose all of it -- there's no cargo-protection mechanic softening that loss the way there is in normal space. Combine that with instant-warp abilities being disabled and it adds up to a simple rule: don't linger with a full hold, and know where your nearest Extract Rift is before you start fighting.
You can travel into the Mirror Universe together with alliance-mates and cooperate once you're there -- there's no restriction against playing as a group. The catch is that Vouchers are spent individually: each player pays their own 300-per-ship entry cost, there's no shared or discounted group entry.