Hey man. Don’t send your camera back. I was having the same exact problem using the lens that shipped with it. I’m new at this too and I realized that I didn’t know how the aperture control worked’ so I thought that I could only fix the darkness by adding a higher ISO (Bad Move).
You change the aperture by holding down the “Av+/-” button while scrolling the wheel in front of the ISO button (Sorry if I’m being captain obvious)
As far as I know when I use the lens that shipped with the camera (18-55mm f/3.5-5.6 IS Lens) it only allows me to lower the aperture to “3.5”, but my 50mm f/1.8 lens gives me the option to lower it to 1.8 of course.
the “Av+/-” button is located near the right top corner of your screen. if you lens is at least an f/1.8 through f/2.4 you should be able to drastically brighten your indoor shots. I’m only stating this because I just figured it out as I am a noob also:)