In 2047, a distress signal is received from the Event Horizon, a starship that disappeared during its maiden voyage to Proxima Centauri seven years before. The rescue vessel Lewis and Clark is dispatched on a rescue mission. On board are Captain Miller and his crew â Lieutenant Starck, Pilot Smith, Medical Technician Peters, Engineer Ensign Justin, Trauma Doctor D.J., and Rescue Technician Cooper. They are joined by the Event Horizon's designer, Dr. William Weir. The Doctor briefs the crew, telling them that the Event Horizon was built to test an experimental gravity drive. The drive generates an artificial black hole and uses it to bridge two points in spacetime, reducing travel time over astronomical distances.
Upon boarding the Event Horizon, the crew finds evidence of a massacre. As they search for survivors, the ship's gravity drive automatically activates. Justin is briefly pulled into the resulting portal. As he is pulled out, he is in a catatonic state, terrified by what he saw on the other side. After he attempts suicide, the crew places him in stasis. At the same time, the activation causes a shock wave that damages the Lewis and Clark, forcing the entire crew to board the Event Horizon.
The team begin to experience hallucinations corresponding to their fears and regrets: Miller sees Corrick, a subordinate he was forced to abandon to his death; Peters sees her son with his legs covered in bloody lesions; and Weir sees his late wife, with missing eyes, urging him to join her. They soon discover a video log of the Event Horizon's crew as they went insane and mutilated each other. The video log ends with a shot of the Event Horizon's captain, who has apparently gouged out his own eyes, holding them up to the camera and saying in Latin, "liberate tuteme ex inferis" (lit. "save yourself from hell"). Miller and D.J. deduce that the ship's gravity drive opened a gateway into a dimension outside the known universe. Weir theorizes that this hellish dimension turned the Event Horizon into a sentient being, which torments its occupants and lures them back to hell.
Miller decides to destroy the Event Horizon. Weir, now possessed by the evil presence, uses an explosive device to destroy the Lewis and Clark. The explosion kills Smith and blasts Cooper off into space. Peters is lured to her death by a hallucination of her son. Weir kills D.J. by vivisecting him and corners Starck on the bridge. Miller confronts Weir, who overpowers him and initiates a 10-minute countdown, after which the Event Horizon will return to the other dimension.
Cooper, having used his space suit's oxygen supply to propel himself back to the ship, appears at the bridge window. Without thinking, Weir shoots at him, breaking the window, and is blown out into space by the ensuing decompression. Miller, Starck, and Cooper survive and manage to seal off the ship's bridge. With their own ship destroyed, Miller plans to split the Event Horizon in two and use the forward section of the ship as a lifeboat. He is attacked by manifestations of Corrick and a resurrected Weir. Miller fights them off and detonates the explosives, sacrificing himself.
The gravity drive activates, pulling the ship's rear section into a black hole. Starck and Cooper enter stasis, beside a comatose Justin, and wait to be rescued. Seventy-two days later, the Event Horizon is boarded by a rescue party, who discover the remaining crew still in stasis. A newly awakened Starck sees Weir posing as one of the rescuers, then wakes up screaming and is comforted by Cooper. As Cooper restrains the terrified Starck and one of the rescuers calls for a sedative, the doors ominously close.

Question: What does Miller and D.J presume to have happened that caused the havoc on the vanished ship?
Answer: A portal opened to an unknown universe and turned the ship into its own sentient evil being