Wealth

A supernova remnant. Maybe you can sell it on eBay.
Image by NASA.

The money in your government's coffers. Money in Aurora is only relevant if you run out of it.

The more in debt the government is, the greater the penalty on production, research etc. And it multiplies with the other penalties. (75% due to overcrowding + 75% due to debt will have the modifier at ~56%).

The current amount is shown in the title bar at the top of the Population/Production window, followed by the absolute change during the last five-day increment. Details are found in the Wealth/Trade tab of that window.

As you build more factories and produce more stuff, your expenses will increase and your overall income will slowly drop. Don't wait until you run of of money.

Things that cost money

• Shipbuilding (each ship or PDC costs an amount equal to its BP value in the Design overview).
• All other construction by factories, including production by ordnance and fighter factories, production of maintenance supplies, and shipyard expansion.
• Buying minerals from Civilian mining complexes will cost you a bundle, once the companies start expanding these complexes. On the other hand, somebody has to dig up the minerals and building mines and mass drivers yourself isn't free, either.
• Subsidizing civilian shipping lines to make them build more ships.
• Research.
• Civilian contracts.
• Ground unit training and maintenance.
• Operation of Maintenance Facilities.

Things that earn you money

• Taxes on population, exports, shipping of freight and colonists, civilian fuel production. You cannot raise or lower tax rates.
• Building Financial Centres. While the name implies banks, they're actually offices of the Inland Revenue.
• Scrapping ships, components and missiles returns tiny sums to your treasury.

Things to do when your coffers are empty

• Stop building things.
• Stop researching things.
• Stop buying minerals. But every ton of minerals you don't buy ends up in the civilian economy, in other words it is lost to you.
• The Construction/Production technology Expand Civilian Economy by 20% does what the name implies - it magically makes your people wealthier and you can mercilessly tax their newfound riches. This tech can actually be researched any number of times, but the research cost doubles each time.
• Set up colonies. They have an initial cost in industry and wealth, but tax and trade make it up fairly quickly. Even just throwing a few thousand infrastructure on a useless world will make a decent amount in trade goods and therefore tax.
• Terraforming can remove the limits to colony growth. Your people will breed like rabbits, happily creating millions of new taxpayers each year. This is a source of constant economic growth.
• Did you notice that some Civilian Administrators have a bonus to wealth creation? Put one of them in charge of a Sector Command HQ for additional effect.