D3100 problem "subject to dark" in a well lit room?

You could always try popping in more light or just ignoring your meter. Good luck.

Okay fine light to your eyes may be dim to the camera. Low light to you may as well be pitch black darkness to the camera already. The camera sees differently. Learn to adjust to it.

When shooting indoors, the general rule is to use flash. If you don't know what you're doing, don't use manual mode.

To shoot in low light without flash, increase ISO. Doing that will make grain or noise more evident in the picture. You will have to strike a compromise as to how much noise you can tolerate just to get a decent shot. Flash is way much easier and you can use the lowest ISO for better looking pictures.

In manual mode, you have to pay attention to the exposure meter on your camera as you are fully responsible to ensure the exposure is correct.

A well lit room is going to be lower in light intensity than outside on a bright sunny day.

What you want to do is watch the exposure meter on your camera as you adjust the combination of aperture, shutter speed, and ISO until the meter is zeroed (centered) on the scale. That indicates the correct exposure.

You're getting that because indeed the lighting is too dark.

There's only three ways to change this. You can up the ISO to 800 or more but then you're asking for noise.

You can use flash and I do understand your reluctance to do that... I don't especially like flash.

Or buy a faster lens. The 50mm f/1.8 is ideal for the darker situations and will probably cure you problem.

Just because you can see clearly, doesn't mean that a camera can, you know the human eye is a marvel. 1/20 at f4.5 is around about right for indoors with no flash.

Of course it's too dark.

Tungsten filaments and fluorescent tubes have a very low colour temperature - your eyes can adjust to compensate, the camera can't.