Where's my money going?

The planets you colonize cost "upkeep"...... try upgrading your planets so they make you money instead of costing you money like your fleet etc. does.

^^

To expand:

Hover mouse over your credit number in the top right, and you'll see which planets are draining your econ. (the numbers will be in red instead of green) This is an underdevelopment tax, essentially a mechanism to prevent mass colonization in the first few minutes of the game. Buying the planet development 'Civilian Infrastructure' removes this penalty. Asteroids need level 1 and planets need level 2. Further levels of Infrastructure on planets are still useful as the population of the planet provides tax income, but the first 2 levels are of emphasized importance. It should be among the first things you buy every time you colonize something.

i dont kno if that is what he is talking about... i will sumtimes notice some of my money disapear as well. i am about to buy sumthing, but then the number goes down as im waiting to click (most noticable early in game when coin is short). i dont kno what it is tho

Pirates have the ability to take chunks of credits from your stash. I think it's a marauder ability.

If your credits are so deep in the red zone that you can no longer get ANY cash, you can scuttle a planet using Alt-H or the caution tape on the bottom of the screen.