...what is it???? It smells like a sewer behind the dishwasher. The pipe is NOT covered and it goes up toward the roof, it's inside the wall, but doesnt come out of the roof. I have no access to the attic to find out where it goes either.
Can I cap the pipe opening behind the dishwasher so it doesnt stink up my kitchen anymore?
That sounds like a vent pipe that must be open inside the frame of the house, instead of being vented to the outside through the roof as required. If you cap it off, whatever fixture(s) it is venting (probably the kitchen sink, dw, etc.) will not drain properly.
You need to open up whatever you need to to get this vent plumbed properly.