Entropy is the first primary currency in the game, and is used to evolve and grow from species to species. It can be created from the very start of the game, simply by tapping on the background of any play area. Over time, as you create new life forms (Generators such as DNA and amino acids) and Upgrades, more Entropy will be generated both in the background and from tapping.
Other forms of currency, like Darwinium, can help out a lot when you're low on Entropy. They can do Quantum Triggers which can give you extra Entropy, usually around the cost of 1 Darwinium. However, if you run out, you would need to buy more, since they are not obtainable through normal play. (Or you can just wait a day to get some as a login reward.)
In the Discord Server, ( https://discordapp.com/invite/celltosingularity ) one of the Emojis is Entropy