The most important things to remember about back story are that (a) everyone has a history and (b) most of it isn’t very interesting.
Once upon a time, I spent copious amounts of time creating back stories for the characters in the novel I was working on. I suspect this was more a form of procrastination seeing as the novel never really got past the first six chapters. Every time I created a new character I'd have to stop and do a character sketch with a detailed back story.
When I finally realized I was spending more time creating back stories than creating the actual story, I stopped. And then I kind of abandoned the book because pretty much all I had was back story, most of which wasn't pertinent to the plot.
It's a fine line between too much and too little when it comes to back story. While you, as the author, need to know everything there is to know about your characters, how much does your reader really need to know? Too much and they'll be bored; too little and they'll be confused.
I did, many years later, revisit that early novel and the back stories came in very handy. They let me get inside my characters' heads so I knew how they'd react in a given situation and made them seem more alive.
Back story can be used to strengthen the reader's bond with your character. It can increase suspense if your reader knows about a trauma your character has suffered in the past that can affect what's happening now. Sometimes back story is needed simply to explain why the character feels so strongly about what they're doing.
In my novel, I used a prologue to share a few important details from all those back stories and more were shown through the thoughts and actions of my characters. The rest appeared through reminiscence and flashback but in total it was only the tip of the iceberg as far as my pages of back story went.
By all means create as much back story as you wish, just be careful how much you actually share with your reader. They neither need nor want to know everything about your characters. Share your facts sparingly, and spread them out. No one likes an info dump, but everyone loves a mystery.