I mean, I saw Space Engineers on Steam and totally fell in love with it, but I didn't want to buy it and then regret in tears. I was wondering, is Space Engineers an online simulation game? Or is there an option where you can play in offline mode or only with your friends?
The Steam Store page indicates that the game is available for Single-player or Multi-player so you should be able to play either way.
Also in the System Requirements it says that no Internet connection is required so you should be able to do offline mode.
Note the game is in Alpha and an Early Access Game on Steam. As with any of those, the game could change these requirements or really any other aspect of the game before release.
It's a local game. As can be seen by the System Requirements, it doesn't need continuous internet connection.
Steam account and internet connection to install the game.
No internet connection required to play the game.
Space Engineer can be played both offline or online.
You can play alone, with your steam friends, or allow anyone to join your game. These are the settings which are available when you start a new game:
When you want a more persistent experience, you can also play on public dedicated servers.