Let's consider the following scenario
A super advanced civilization decided, some time ago, to send a fleet to explore the galaxie

Some figures

Alien fleet : 10.000 stars ships
Cruise speed : 10 % of the speed of light
crew : 100 members
average life span : 1.000 years

Mission : To explore 1 % of our galaxi
Expected time to complete the mission : 40.000.000 years
Human resources : 4.000.000.000 people, living 100 % of their life on board

You can make more sophisticated simulations, including for example the foundation of colonies but you will alwayes obtain non realistic scenarios

No seriuos scientits beleive that aliens visited our earth because of the size of our galaxie and the speed limit of light
The average distance between a star system and the next one , in our galaxi, is about 10 light years
A star ship would take 100 years to jump from one star to an other one
The number of stars is supposed to be about 400.000.000.000

An other matter is the existence of aliens

There are 400 bilion galaxies in the universe
Very likely aliens do exist