I'm trying to find a way to start games using Galaxy in silent mode.
I know you can run a game without Galaxy, but I understand some features like achievements and online gaming are features provided by Galaxy and I don't mind running it. What I do mind is opening the launcher.
I use a dedicated PC with a controller using Big Box as the launcher. The problem is that I want to start games without opening Galaxy launcher, but keeping its features.
I found no documentation at all about its command line arguments, which is kind of bizarre, and the best command I came up with was
"{GalaxyDir}\GalaxyClient.exe" /launchViaAutoStart /command=runGame /gameId={GameId} /path="{GameDir}"
It almost works. It starts in a silent mode, but only if Galaxy is not already running. The next time I start a game, the launcher is opened.

Does anyone know a way to do this?

So by "silent mode" you just mean you want the main window minimized or hidden.

Maybe use a third-party application to hide the window? In a quick test, I was able to hide it with NirCmd, but I can only get it to show again by clicking the system tray icon.
nircmd win hide process GalaxyClient.exe

Edit: the commands for minimizing and restoring the window work fine (the one to toggle the minimized state doesn't).
nircmd win min process GalaxyClient.exe
nircmd win normal process GalaxyClient.exe

You can also try the exec command to start directly in a minimized state.

Exactly. The main problem is that I cannot close it using a controller. The ideia is for the launcher to not show, just use it for the features that it adds to the games. I was hoping for a more "natural" or native way to do it in Galaxy.
I might try a custom Auto Hotkey script for that as Launchbox already have support for it.
I actually have no intention of showing the Galaxy launcher as the idea is to use Big Box. But that NirCmd tool seems to be a nice option too.

You can just launch the game executable, even with Galaxy closed it will use all the features.

I was doing exactly that, but achievements were not being tracked, for example.