Why does my TV look so dark?

Dark scenes you cannot see into, a picture that dims by itself, HDR that somehow looks worse than normal video. A dark TV is almost never a broken panel. It is a handful of settings fighting you, and every one of them is a two-minute fix. Here they are, in the order worth checking.

First suspect: The energy saver

This is the culprit far more often than anything else. Nearly every TV ships with an eco or power-saving mode that reads the room with a light sensor and quietly pulls the brightness down, hardest in dim rooms and during dark scenes, which is exactly when you want it up. It also tends to reset itself back on after an update.

Find and turn off anything named Eco, Energy Saving, Power Saving, Ambient Light Detection, Auto Brightness or Brightness Optimisation. It usually lives in a General, Eco or Power menu rather than the picture settings, which is why people miss it.

The picture mode and backlight

Two picture settings decide most of the overall brightness:

Picture mode. Eco and some Standard modes cap brightness on purpose. Switch to Cinema, Movie, or, for daytime, a Standard or Bright mode. Filmmaker mode is the most accurate but also the dimmest, and it is meant for a dark room.
Backlight or OLED Light. This is the real brightness control, and it is separate from Brightness (which sets black level, see below). Turn it up. On an LCD it is called Backlight; on an OLED, OLED Light or OLED Pixel Brightness.

Crushed blacks: When dark scenes lose all detail

If the overall picture is fine but dark scenes turn into featureless black holes, the Brightness control is set too low. Confusingly, Brightness on a TV does not set overall brightness; it sets the black level, how deep the shadows go. Too low and shadow detail is swallowed into black. Set it with a black-level pattern: Raise it until the near-black steps become visible, then lower it just until true black looks black again. The Calibration Patterns include a black-level pattern made for this.

Why HDR can look darker, not brighter

HDR is supposed to be brighter, so it is jarring when it looks dimmer than ordinary video. A few reasons:

The room. HDR puts more of the picture into deliberate shadow, and reserves brightness for small highlights. In a bright room those shadows just read as murk. HDR rewards a dimmer room.
The panel cannot reach the brightness the content asks for, so it maps everything down and the mid-tones sink. A brighter TV suffers this less.
The HDR picture mode is set to a dim preset. Look for an HDR-specific mode (HDR Cinema, HDR Bright) and raise its backlight.

Worth checking that HDR is even engaging properly in the first place. The TV Capability Test reports whether HDR is being signalled to your screen at all.

Questions people ask

Why does my TV keep dimming by itself?

An ambient light sensor. The TV is reading the room and lowering brightness to save power, and it is overdoing it. Turn off Eco, Energy Saving, Ambient Light Detection or Auto Brightness, usually found in a General or Power menu rather than the picture settings.

Why is HDR darker than normal video on my TV?

Two things, usually. HDR deliberately keeps most of the image dim and spends its brightness on small highlights, which reads as dark in a bright room. And if the panel cannot hit the brightness the content requests, it tone-maps everything down. Try a dimmer room, raise the backlight in the HDR picture mode, and make sure you are not in an Eco mode.

Why can I not see anything in dark scenes?

The Brightness control, which sets black level, is too low and is crushing shadow detail into black. Raise it with a black-level test pattern until the darkest details reappear, then ease it back so black still looks black rather than gray.

To set black level and contrast properly by eye, use the Calibration Patterns. To check what your set actually supports, including whether HDR is really being used, run the TV Capability Test.