You can pick up Negative Mass from the scrap shop on Sithyrya.
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They really should have ship fuel in akra. It's weird Warlord's Sythiria having it and Akra not, after all it's stated many times that Sythiria is not as technologically developed than Akra. Also, there's way more ships travelling to and from Akra, after all, no one wants to go to a place where a war with a immortal god is going on.
Devs, please, add negative mass to the grosseries in Akra. Or add Katari's old boss shop as a place to buy scrap, ship upgrading materials and fuel (and other tech related stuff)