There is no explanation given as to where the aliens originated from. Are they a result of meteor strike that I could see it on a piece of newspaper on his board?
Their planet was exploded and resulted in a meteor strike. So, the question arises "how did they survive this explosion". Well, it is because they have a shield making them almost invulnerable.
From this excerpt of the podcast with John Krasinski (emphasis mine),
They are absolutely aliens. They’re from another planet.
The idea behind all that is they’re definitely aliens and they’re an evolutionarily perfect machine. So the idea is if they grew up on a planet that had no humans and no light then they don’t need eyes, they can only hunt by sound. They also develop a way to protect themselves from everything else so that’s why they’re bulletproof and all these things. I had to make it make sense. I needed the rules of the monster to adhere as tightly to the rules of the family. The family, we had set up all these incredible rules, and I needed the monster to not just be convenient.
And the other idea was [the armor is] also the reason why they were able to survive kind of the explosion of their planet and then survive on these meteorites, because they’ve evolved to be bulletproof. Until they open themselves up to be vulnerable, they’re completely invulnerable.