(SPOILER WARNING - I will be discussing events and characters from The Force Awakens)
One of the largest plot points of the prequel trilogy was the prophecy of The Chosen One, believed to be Anakin Skywalker, who would bring balance to the Force. It has been interpreted in a number of ways, personally I always saw the balance as being an equal amount of Force Sensitive beings on both the Light and Dark sides of the Force.
It is considered that he succeeded in bringing this balance when he killed The Emperor before dying himself. Leaving Luke as the only known Jedi in the galaxy. However, this would mean that somewhere there was another being that aligned themselves with the Dark side of the Force.
That being was revealed to be Supreme Leader Snoke, a powerful creature that rose to oversee The First Order. Whilst we do not know quite how strong his force capabilities are, we know that he is at least very capable at manipulating the minds of others. So presumably a balance was achieved but what would happen when a new generation of Force users arose?
(FINAL SPOILER WARNING)
Maintaining The Balance
By the events of The Force Awakens, thirty years after Return of the Jedi, two new Force Sensitive characters are now present, Rey and Ben. While we know Ben's parents are Han and Leia, we currently do not know who Rey's parents are. Although my money is on her being the daughter of Luke.
Both started off on the Light side of the Force, but Ben was eventually seduced to the Dark side by Snoke. This was a moment arguably destined to happen, so balance remained. Ben transformed into Kylo Ren, discarding his true name and hiding his face behind a mask.
(Side Note) This is why I believe the confrontation between him and his father couldn't have ended differently because as he Kylo said himself Ben was dead, his past-self tried to pull him back to the Light, but for balance to remain, he had to destroy this temptation.
For most it would appear that the sides are balanced however what about a character many seem to forget is Force Sensitive?
What About Leia?
While most remember that she is Luke's sister, few ever properly consider what that brings to the table in terms of balance in the Force. Based on our current knowledge of characters within the universe there is three Light side characters against two. So who could this last member of the Dark Side be?
Snoke and Kylo seem to have the classic master and apprentice relationship so it is unlikely to be another person directly connected to them. Also assuming the balance was achieved upon the death of Anakin, it must be someone who was at least around by Return of the Jedi, ruling out the children of any other Original Trilogy characters.
So that leaves us with someone at least 30 years old, who was already aligned with the Dark side of the Force and is powerful enough to be a threat to our heroes. Based upon a certain line that Snoke says I have an idea on who it could possibly be:
An Ancient Evil
Quite a few theories have pointed towards the idea of Snoke actually being Darth Plagueis, the former master of Emperor Palpatine. I believe however that they are two separate entities, one of which lied dormant regaining his strength after his apparent murder at the hands of his apprentice.
Given what we know of Plagueis' power, from Palpatine's description of him, he had the power to revive those who had died and stop potentially death itself. Unfortunately, as Palpatine claimed, he was unable to do the same for himself and was killed by his apprentice.
But what if he could? Even if he wasn't entirely aware of it himself. Considering Snoke spoke of an 'awakening' in the force, what if it truly was a being previously resting that has once again risen? One strong enough to send a ripple through the force with enough power for others to sense.
After all, it could also explain why Luke decided to reveal his location to the others. To come out of hiding to confront the next big threat to the galaxy.