I like the game, a very scientific approach of an incremental/idle game. It's fun finding an equilibrium of fuel/engines and improving it with upgrades.
The Lorentz Factor is a dimensionless variable, i. e. it has no unit instead of m/s. This is an actual error, the following feedback I'll write is just personal preference.
The start is a bit rough imho, there are a lot of numbers. I mean all of them make sense and have their purpose, but still there are many numbers at start. Maybe hiding numbers like Lorentz Factor or Proper Velocity until they actually become relevant later in the game could help.
Antimatter Engines are worse than Fusion Engines, it takes such a long time until they get better than Fusion Engines. It feels a little bit too slow for me. After researching Antimatter Engines you just have to invest so much more to actually get them relevant.
This game also needs better number representation. It's so difficult comparing the Output or Thrust of different engines, you have to count like 17 or 18 digits. SI prefixes or exponential notation would help significantly.
Overall it's a nice game, I enjoyed playing it.