Hi!
WOW so many different answers!
+1!
So first of all you shouldn’t use too dark materials because as you see you’ll get unrealistic results!
You can almost always brighten up textures/materials inside Unreal!
You didn’t say anything about your lighting… is it static or dynamic…
With dynamic lights you won’t get GI so if you want realistic (and not faked) results you’ll need to use those…
I’d never change diffuse boost or indirect lighting intensity: they both tend to cause errors and your lighting will be off! /have seen a LOT of errors/problems because of those!!
What I would do is set your material brighter (try the unlit view mode and you should see a flat grey(ish) look!).
Then set your static/stationary lighting properly (with portals, Lightmass Importance Volume…). In your World settings under Lightmass I would go somewhere between 5-10 with both skylight and indirect lighting bounces the rest of the settings are up to you what quality you’re after…
You can adjust a lot with post processing! Brighten up the scene for one!
Well this is for a start… To be honest you shouldn’t have a problem to light up a scene like this in Unreal…