Per default the live view tries to show what the exposed photo will look like. I.e. if you, in manual mode, select the smallest aperture and a short shutter time and subject you shoot is in low light, your photo will just become black/dark. In this case the live view will also be dark.
This function can be changed so the live view is always looking nice and balanced.
I found the following in the users guide for my A77ii. It is probably about the same for most cameras with live view (also had the same trouble with a C*non at work lately):
Live View Display
Sets whether or not to show images altered with effects of the exposure compensation, white balance, [Creative Style], or [Picture Effect] on the screen.
1. MENU → (Custom Settings) → [Live View Display] → desired setting.
Menu item details:
Setting Effect ON (default setting):
Displays Live View in conditions close to what your picture will look like as a result of applying all your settings. This setting is useful when you want to shoot pictures while checking the results of the shot on the Live View screen.
Setting Effect OFF:
Shows Live View without the effects of exposure compensation, white balance, [Creative Style], or [Picture Effect]. When this setting is used, you can easily check the image composition.
Live View is always displayed with the appropriate brightness even in [Manual Exposure] mode.
- When [Setting Effect OFF] is selected, the icon is displayed on the Live View screen.
- When the shooting mode is set to [Intelligent Auto], [Superior Auto], [Sweep Panorama], [Movie] or [Scene Selection], [Live View Display] cannot be set to [Setting Effect OFF].
- When [Live View Display] is set to [Setting Effect ON], you may have difficulty checking the composition, as the Live View Display may seem too dark or too bright. If such a phenomenon occurs, set [Live View Display] to [Setting Effect OFF].
- When [Live View Display] is set to [Setting Effect OFF], the brightness of the shot image will not be the same as that of the displayed Live View.
- When you use a third-party flash, such as a studio flash, Live View Display may be dark for some shutter speed settings. When [Live View Display] is set to [Setting Effect OFF], Live View Display will be displayed brightly, so that you can easily check the composition.