Stephen Hawking's Demons

Robotic-sounding voice. I am Steven Hawking. I am a human being attached to this cyborg time-traveling unit. You're hearing my voice from a voice synthesizing computer in my helmet. You may not understand, but suffice to say I am from the future. I am a great scientist to the people living there. I have understanding of technology that could change your civilization forever. For example, it's a property of matter to continue in a straight line until its state is changed by an external force. We're all affected by the gravity of earth, so its not entirely obvious to you, but it's true. You'll find the knowledge useful for controlling rockets to space. The space race is coming 400 years early, priest. We're going to save earth from the creatures made of dark matter.

Listen, you. Inertia to them works entirely backwards. In the world of dark matter, everything is a state of total chaos until interacted with, at which case it takes a solid form. For the dark matter beings, time stands still whenever they're influenced by external forces. Otherwise, they're in a state of total chaos, which is a form unrecognizable to us who are made of matter. 

"Take me to your highest eaves, priest. I must scan the horizon and sky for signs of satellites. I have a theory the reverse-inertial beings were present during this era."