I believe that there intellectual life not only in the universe, but also in our galaxy. There is no way that Earth is the only planet that has evolved intelligent life. However, the odds of that life living long enough to develop interstellar travel I believe is rare.
If a civilisation capable of intergalactic travel came into contact with Earth, it's really impossible to say what the overall impact would be.
First, for the people of Earth, it would be pretty bad to begin with. There would be without a doubt mass panic and global economical collapse at the very least. Religion would also suffer. How could religion make sense after that sort of event?
The Aliens themselves........it honestly would be impossible to say or predict what their intentions would be.
There is a sci-fi trope that the Aliens would come to harm us. Maybe wipe us of the planet and harvest our resources and move on. Stephen Hawking said that it would be the equivalent of European nations finding America, which didn't turn out well for the natives.
Some say the Aliens would come in peace. If they have the tech to travel the galaxy, why would they come to harm us? Maybe they would come to help us, gift us technology and turn us into a utopian society.
However, both instances make the mistake of projecting our world view into that of these Aliens. Humans are by nature violent creatures. Our whole history has been shaped by war and conflict. We also can on the other hand show greats amount of empathy to our fellow humans.
I don't think any Aliens would be either. I can't say what their intentions would be, but I believe their intentions would be completely foreign to us and wouldn't make any sense at all. Their intentions would be absolutely alien. Pun intended.
They may, for example arrive at earth, say absolutely nothing, deploy a biological weapon that kills two thirds of all human life and then disappear never to be seen again. For years afterwards, humans that are left would be discussing and debating the point of such a cull, but the to the aliens who did the deed, it would make perfect sense in their alien minds.
I'd also like to apologise for the above if it appears to be the ramblings of a mad man. I've just drunk a bottle of Italian red wine and and pretty wasted.
Cheers!