Aliens want the colony's infinite-energy cores. Corporate would prefer they didn't.
Dark Matter is earned from every mission, win or lose. Nobody knows what it is. Finance says it spends fine.
The aliens are also on break. Union rules.
Aliens walk to the vault, grab a core, and head back to the portal. Kill a carrier and it drops the core, which floats home. Towers are walls: make the route long. Lose every core and it's over.