The ever present ominous unknown threat(s) is what keeps this movie on the edge of your seat. You never knew what's around the next corner, and uncertainty is a constant.
Imagine if you will, or can, you find yourself waking up to that news. How would you react? What extent would you go to survive and protect your loved ones? This movie touches on those aspects.
Because it did end open ended instead of giving any semblance of closure is why I gave it only 4 stars. I find the likelihood of a sequel to provide this closure slim to none as most people don't enjoy using their imagination to fill in the missing pieces.