If you need to make your short story or novel longer, every writer faces the challenge of writing quality along with all of that quantity.
The writers who fail in this task are accused of 'word-padding', which is also known as the meritless use of extraneous and lengthy words with the stated goal of increasing one's page count and giving the impression of competence as a provider of serious-- which is to say long --literary works.
See what I did there? Please don't do that with your stories. Your writing should either:
1. Advance the plot,
2. Endear a character,
3. Make a villain infamous, or
4. Establish an ambiance.
A way to accomplish all four of these goals at once is to incorporate a backstory into a conversation between or among characters. Ideally, you could introduce your protagonist and his or her sidekicks, friends, coworkers, or family members by having the villain's backstory come up in conversation. This has a few benefits.
First, it introduces your main characters, gives the reader a sense of what upsets the characters, and tells the readers why they should support the main characters against the villain(s).
Second, it allows you to tell the villain's backstory without resorting to the long-winded and arrogant tone, such as: In the dark land of *insert ominous name here* the dastardly and feared Lord/Ruler *ironic or fitting name here* perfected his wickedness against the innocent and helpless masses. *Insert long and dreadful list of atrocities.* Who would stop him?...*Proceed to first chapter, with idyllic hero figure*
An example of how to introduce your villain and hero without resorting to boring exposition is this. Consider a science fiction story, where all the characters must cooperate on a decades-long mission on a space ship. Your main character and his friends could be having lunch in the cafeteria, and discuss--over their bland rations--how pushy and irritating the ship's psychiatrist is. "He's supposed to help us acclimate," one crewmember might complain, "not make us consider using an airlock without a space suit to escape a session with him!"
Have the characters argue whether the psychiatrist is actually that bad, then discuss what--if anything--they can do about it, and decide whether to confront the psychiatrist directly, mention the problem to the ship's captain, or refuse to attend the mandatory sessions. No matter what the decision, it will convey where your characters stand, how bad the villain of your piece is, and give a sense of what limited options are available to them as the plot of your story progresses.