Franklin Richards (Sue and Reed Richards' son) was jealous of how much time his parents where spending with his sister. Wanting make himself smarter, he messed around with his dad's personal digital assistant. Because it was connected to another dimension (to give it infinite storage), he unintentionally created a living math problem (made out of electrochemicals) called "Modulus". The being wanted to literally bond with Reed. It started to try to murder everyone else that Reed shows affection to. Sue managed to save herself and the Thing. Reed discovered that Modulus was actually half of a math equation hence why it felt lonely. As such, Reed created a living math problem copy of himself. He gave it to Modulus, who returned to his own dimension with the copy.
Modulus had various powers. He has super strength and invulnerability. He could reshape his body and manipulate energy. He also could "[divide] molecules... to reduce them into chemical components" (Fantastic Four: The Ultimate Guide).
Fantastic Four: The Ultimate Guide section "Modulus"