Science can maybe explain all that came after the was created and possibly how it came into existence, however, what has it explained about what was there before? Nothing, this is where religion came in and theorized or believed that God created the universe. If you say that God is impossible you may as well say that human existence is impossible because how can everything, come from nothingness?
I'm not denying that your theory on fairies is invalid, however you lack sufficient proof to show that they're real and you're also lacking substance to the argument that "God is impossible." Life after death is another matter, you cant deny it because no one can tell you what it's like after death. Furthermore I'll reiterate that a fairy having magic would need to be created by a supernatural power as there is no natural reasoning nor a scientific reason for this to happen, thus meaning that if a fairy can exist through supernatural circumstances, then a God can.