I have theories on what the entities are.

Rating: 8/10 (5 January 2019)

Bird Box review by The Foiler
3 Stars, out of 4, is my official rating. (I typically don't do 5-stars or 10-stars basis, unless it's necessary)
Let me get this hot item out of the way first, before I get to my review:
I like originality. So yes, I get it that people are criticizing it for ripping off "A Quiet Place" and "The Happening." But if you look at it another way, most top-tier movies have taken ideas from other movies. The critically-acclaimed Star Wars 1977 was not 100% original. The opening crawl was taken from the 1930s Flash Gordon TV show's opening intro. The giant boulder that chased Indy in the greatest action-adventure film in history, Raiders of the Lost Ark, came from the 1950s Duck Tales comic strip, in which a similar boulder chased Uncle Scrooge after taking a golden idol. The face-melting scene at the end of Raiders was also taken from the ending of 1955's Kiss Me Deadly. I can go on and on. So, I wouldn't write off Bird Box's premise just because people think it's a rip off.
Now on to my review. What I think these entities are.
SPOILER ALERT
I've always wondered why these extremely powerful beings cannot break inside homes, but they are deadly outside. I think it has to do with the open sky. Inside your home, there is no open sky, right?
So, what is present in the open sky?
Satellites!
Theory 1: It's a man-made entity that was weaponized by a warring nation, whether it is China or Russia. Heck, it could be the States. It's a bio-chemical compound that was released into the atmosphere, and it gets its energy when a signal from the ground is sent up to the satellites, and then beamed down to give these chemicals the needed energy. Once it receives the energy, it becomes potent. In other words, when you look at it, it affects the retina, and then affects the part of the brain that triggers suicide.
Theory 2: It is not man-made. It is alien in nature. But like Theory 1, it still needs open sky in order to trigger the entities' potency. A signal is sent from outer space that gives energy to the entity, but this signal cannot penetrate solid objects. It could only penetrate through air molecules and the vacuum of space. Which is why you are safe inside your home.
Theory 3: Nothing to do with open sky. These are just creatures that are so weak that they cannot break through doors. But if you are outside with them, they have the power to destroy you.
Think of pit bulls. Say, there are 10 hungry pit bulls outside your house. And you are outside with them. These animals will maul you to death. But if you're inside your home, you can have a thousand pit bulls outside, and they won't be able to break in to your house.
Overall, it's a nice film. Great acting by Bullock and Malkovich. Good pacing: it was suspenseful throughout the entire movie.
Reviewed by The Foiler, on 4 January 2019
"I will foil your evil plans."