Your camera meter is doing what it is designed to do: it tries to render the metered scene as a midtone (roughly 18% gray). In a very dark room, that means the camera tends to brighten the scene, so a “correct” 0 reading can make the photo look lighter than reality.

To keep the room looking dark, deliberately underexpose relative to the meter reading. You can do that by:

• using negative exposure compensation (for example, around -1 to -2 EV), or
• switching to full Manual mode and setting a darker exposure yourself.

Metering mode also matters. Average/matrix-style metering looks at the whole scene and will often brighten a dark room. Spot metering can help if you meter from a specific area and decide how dark you want it to appear.

A practical approach:

• Use Manual mode.
• Turn off Auto ISO and keep ISO as low as practical.
• Use a tripod if shutter speeds get long.
• Set your aperture as desired.
• Start from the meter’s suggested exposure, then shorten shutter speed until the image matches the mood you want.

The key idea: the camera cannot know you want the scene to stay dark—you must tell it by exposing below the meter’s “normal” value.