Artificial sugar has no carbs, this should be a good thing right? Nope, tongue tastes something sweet, brain says, "Ah ha, that sure is sweet. It must have carbs, carbs are energy we got to have a plan for this energy. I know just the man for the job, do your thing Pancreas."
And the Pancreas is like, "Ooo! Digestive hormones, that's kind of my thing... this calls for some insulin! You go little guys!"
And so the little insulin bunnies start hopping around, and they're going through the system and making it all ready for any excess energy and all that. And the body isn't burning the energy so the insulin bunnies decide to hop around and make fat cells to store the non-carbs. But there are no carbs to store and the bodies like, "Dude, what gives. You've got these empty fat cells, go fill them up."
This makes the people hungry and they have to eat real food. Only when they do that the body makes twice as many fat cells so if you mix artificial sweeteners with real carbs, you'll actually become fatter than if you went without the artificial sweeteners.
This is what insulin looks like.