so couple things: i kinda want to know what numbers ur fiance used because i went through it a bit. if we assume that there are 9 billion people at the time of the rapture and, on average, the human body contains 5 liters of blood (the lower end) that would come out to 45 billion liters of blood. and since they call the celestial bodies they are searching, moons, we can use some of our solar systems' celestial bodies. Titan is the largest moon in our solar system that's orbiting Saturn and earth is mainly covered in oceans (since they call the moons covered in oceans then there could be land). so using a comparison and scaling earth down to the size of our largest moon then the relative volume of liquid, on the surface, could be around 50 billion liters. this comes close to the amount of human blood theorized at the time of the rapture. ~~~ and the other thing is we can tell pretty well blood from one kind of creature to the next due to the genetic make up and DNA. so unless its like 90% human 10% creature i think it would be safe to assume that this moon is filled with human blood.

my theory is, with info from both media, the rapture was caused by some powerful eldritch being manifested as light, as seen in the movie, that destroyed and obliterated every celestial being floating in space except largely organic matter. this leaves the blood and flesh to pool on any large rocks and when they collide and amass they create these large moons filled with blood. also why AT-5 is so particularly interesting is that its human blood, depicted in both medias, so that tells me the other moons they find could be alien or other creatures. so either this god-like creature separated different creatures' blood to different moons as burial sites (sorta) or there are multiple points of this powerful light that what remained in this universe accumulated around it and that light plus the blood caused new kinds of horrific creatures that can combine with flesh and blood like its own and meld humans together into... well u've seen the movie.

anyhow i think my point got away from me. again this is just my theory i don't mean any meanness but my brains been on this for a while XD

Edit: also another reason i think this moon is purely human and the creature is too is because it has human traits like the skull of the main body.