In almost every science fiction movies, the galaxy is filled with so many alien races. We depict them as invaders pillaging our planet for resources.

But is that true?

We cannot be sure, since we have no definite proof

The milky way is very big, so massive that there are billions of different star systems.
To answer that question, we have to look at humanity itself.
The reason we even exist.
There are many theories out there trying to explain how humans evolved to what we are today.
We started as simple minerals combining together creating cellular life forms or simply bacteria.

To get to that point, there are certain variables we have to look at.

Just the right distance

There is something called the goldilock zone or habitable zone.

Luckily earth is at just the right distance away from the sun and thanks to our ozone layer, we will not immediately die from too much sun exposure.
To support life, a planet has to be in that zone or it will be either too cold or hot for life to evolve.

The planet itself

Even if the planet is in the right zone, the sun can still be a danger. Thanks to water evaporating (and other elements) and earths gravity, an atmosphere has been formed, with that came the ozone layer.

Mars for example, has an atmosphere, but it would be too dangerous to live there without a suit, since the sun's radiation is still too dangerous, because that layer is too thin.
Even with an atmosphere, what do we need most?

Exactly

We need Water, but that is what the planet already has.
Like I said in the beginning, simple cellular "life" forms were created by combining minerals.

Evolution

Now that the planet is filled with germs, all they need is to evolve.
Easier said than done. Evolution can be slow or fast at times, it depends on outside forces.
Evolution means "adjusting", it would take a long time to explain it thoroughly, so let's just keep it simple.
Bacteria multiplied and created the first "sentient" animal.
Since life formed in the ocean, these animals developed the ability to "breathe" underwater.
Many thousands and thousands of years later, after creating different fish, getting bigger and bigger.

As I reread what I have already written, I realize my post is getting a bit out of hand and I am deviating from my original topic.

So, I will just say this now. To have these exact variables, is pretty rare, there may be billions of star systems in our galaxy, but to find earth-like planets WITH sentient life, is rare.
Also, it is unknown if those alien races are even technologically advanced.
Maybe if we visit them sometimes, they would consider us gods?

I sure do hope that I will be able to answer that question with 100% certainty.

As of now, have a nice day.