Why can't I take photos in darker places without flash?

Just use the flash init fool

Up your ISO to it's highest setting and use the noise filter.   I hope you are using a monopod or tripod, or you won't have a chance of getting any tack sharp photographs.
Good luck.

Not a big flash fan, that'll cost ya. Read on...

I'm just going to answer the last two questions. You have to have what are called fast lenses. These are lenses like the Nikkor 70-200 f2.8 ($1200) that make this possible. See my site gallery for examples. I will either do manual or sports setting, it depends on the amount of ambient light. I can't use flash, it's not permitted in my work. Thus, the fast lens f2.8 all the way through the mm range, is the only lens I can use to achieve this. Some lenses (less expensive of course) are rated and medium lenses, that is, at 70mm, they have a low f range of 2.8 and at 200 mm have a lower range at 5.2. Ouch.

That's simliar to squinting to see the results in a dimly lit room. Whereas the f2.8 lens through the whole range is like having your eyes wide open in a dimly lit room, no matter how much extra light was around.

In short, in order to not get a blurry picture, because of movement, you have to have your shutter go off above 1/250 or 1/500. But to get that you have to have enough light or the image will come out black. Thus, to compensate for both of those, you have to have a lens that is "fast" enough to read the light and yet stay high on the shutter speed.

Again, it'll cost ya.