People who find it difficult to understand the concept of digital currency and who failed to invest in bitcoin currecy when it was cheap are calling it a bubble. When any currency is manipulated by a group of people for their personal benefit the unnatural and sudden increase or decrease in such curremcies is called as bubble. Bitcoin is a decentralized currency so such manipulation is impossible.  Bitcoin is increasing continuously since last two years. And even after crossing $19000 mark, it is rolling down slowly not suddenly. It shown bitcoin is not a bubble.