New DSLR sigma lens 18-105mm, very dark photographs?

Hi, I'm a relatively new to photography, so please bear with me. I have been using the Nikon 18-55mm kit lens that came with my D3100.
Recently, I bought a new sigma lens, 18-105mm. However since using it, my photographs became very dark.

I shoot in aperture priority mode & in the same room with my 18-55mm lens, with an f-stop of 5.6, & ISO of 100 the shutter speed required would not be so slow, & allowed me to capture the room well lited.

But with my new lens, with the same f-stop of 5.6, the shutter speed would be about 1-2s in the same lighted room. Why is this so? Is it a camera setting or the lens? Please help me & once again, I'm still relatively new to DSLR lens & equipment!

Make sure the lens is mounted properly. If you're still having problems, take it back where you got it and explain.

The light gathering power of the Sigma lens is less than that of the lens that came with the camera.

Just raising the ISO a bit should cure your problem. Raise your ISO to "200" and check the reults if still too dark than go higher. We can't tell you specifically because it would be impossible to relate the exact lighting brightness to us.

With the new lens, even with a slower 1-2sec, it is still darker than the Nikon lens?