If anyone ever tells you there's no such thing as monsters, they're lying. There are monsters all over the place. It's just that we've gotten quite good at ignoring them, banishing them to the outskirts of civilization, or remaining ignorant. Children understand this. When they are afraid of the dark, it's because they know that the biggest, meanest, most dangerous monsters are invisible; and what is the dark but a very commonplace version of invisibility?
I suppose you want some examples of monsters. That's good, because that's what I was going to tell you next.
Category 1: Monsters We Ignore
You know that guy who cuts you off? Or the one who reads their receipt at the cash register with eight people in line behind them, finds a $1 mistake, and makes the cashier call a manager immediately? Or the lady in front of you who drives 35mph exactly in a 40mph zone, and is going exactly the direction you are? These people are not monsters. But they have little monsters inside their heads. Little monsters that whisper "who are you?" and "why?" and "RAWR" and "you're stupid". These little monsters can have devastating consequences on us. Sometimes they're even contagious. But usually we just ignore them.
The second type in this category are monsters such as The Runny Nose, The Hang Nail, and The Sneeze. These are constantly attacking, but we've developed complex defense systems that allow us to ignore the monsters most of the time.
Category 2: Monsters We Banish
These monsters usually take forms of bears, mountain lions, volcanoes, and poisonous reptiles. Typically we keep them at bay by installing electric lights everywhere, shooting at them if they come to close, or destroying their food sources and habitats. These ones are probably the least dangerous and the most feared.
On the other side of this coin is the Germ. The germ is tiny, but it invades our body and slowly takes out one of our functioning systems at a time. We have developed many defense mechanisms for this one; the main one is banishment through vaccinations.
Category 3: Monsters We Are Ignorant Of
This is a bit of a difficult category, because if we're ignorant, it means we have no knowledge. However, I at least know of one: black holes. You think your teenage brother eats a lot? My little brother would wrack up a $400 grocery bill all on his own when he was thirteen--a black hole would eat the whole planet if it could get its greedy little hands on it. And that would just be the beginning. A black hole would eat our sun as well as all of the surrounding stars. It just sucks in everything, like a giant vacuum--it eats stars, planets, moons, dust, asteroids, entire galaxies. Even you. The worst part is that they'll eat light: mixing your wildest fears into one terrible creature: a giant, ravenous monster that consumes everything, in the dark. Not only would it be consuming you, you wouldn't be able to see it.
So just watch out for little monsters and big monsters, because they're sneaking everywhere, stealing your socks and your hair, making you grumpy and scratching your car, and the only way to fight back is to stomp on them. Except black holes, you can't stomp on those.