What is dark energy (the fifth force) ?

What is dark energy ?

Dark energy is also known as the fifth force because it is totally different from the four fundamental forces present in universe.

Image credit : Cosmos Magazine

 The fundamental forces are :

• Gravitational force
• Electrostatic force  
• Weak nuclear forces
• Strong nuclear forces

The fifth force 
Dark energy is reffered as the fifth force. Because it has different properties than all other known forces.
As we know that the universe is expanding apart , it is constantly under the effect of dark energy. This is a force which counters gravity.
 Gravity is a fundamental force which acts between two masses. So it must be also acting between two galaxies but most of the galaxies are moving apart even after gravity being an attractive force. 
So we must consider a fifth force into action and it is called dark energy. 

Where this force is overshadowed by gravity the galaxies starts moving close like Milky way and Andromeda galaxy. The milky way andromeda collision. 

What do you think about the fifth force ?