The main currency used in known space is the Energy Chit, backed by Galactic Energy Supplies. A single Energy Chit can be exchanged for one standard Charge of energy at any GES installation - this guaranteed (and politically independent) base makes the energy chit extremely stable, so it is an excellent currency for those who trade with more than one Faction.
Although individual worlds may make use of their own currencies, backed by other models, the ubiquity of the energy chit means that it is rare that anything else would even be offered. Energy chits are used for the vast majority of transactions between space travellers, factions, and businesses that operate on more than a single world.
Although energy chits are traded virtually, they are also issued as physical currency in decimal denominations. The hard polymer physical chits are implanted with an identification matrix that makes them nigh-impossible to forge. They are durable, and although they are sized consistently, they are distinguishable by both colour and pattern. For ease of identification, each denomination is a different colour, cut with a different pattern, and with a number of central holes indicating the number of digits in that chit's value.