Here's a new feature I'm really excited about : the visibility / stealth system. Detecting enemy ships is done by scanning for electromagnetic radiations with active or passive sensors. The active sensors emit EM radiations in the hope it will bounce on enemy ship back to the sensor, effectively detecting ships but also decreasing your own stealth as it emit radiations. Passive sensors detect radiations emitted or reflected by the enemy ships without emitting radiation itself. This one highly depends on the target own emissions and it's reflection of the nearby star emissions.
Also, some range of the EM spectrum have more specificusages. Radio wave, infrared and light radiations are mostly similar. But X-ray can detect ships hiding in dust clouds and Gamma-ray detects ship warp in/out.
When a unit fires or warps it will emit additional radiations, that kind of actions usually result in being detected.
But the ultimate in scanners are the neutrino sensors. Neutrinos are weakly interacting particles emitted by most ship reactors.
Meaning they tend to go through everything, making them great for scanning through asteroids field or high density dust cloud. However the same property that makes them so efficient also cause the detector to be quite huge as the particles don't interact much with it. Thus you will need a huge ship to carry one such detector in your fleet.
So if you want to go on a stealth mission, use passive detectors, low emitting reactors and paint your ship black like a ninja!
And finally, here's a preview of what the fog of war will look like. The bright bubbles are regions covered by your ships' sensors.