Black swan event: refers to a very unpredictable and unconventional event, which usually causes negative reactions or even subversion in the market chain. In index funds, this means that the investment risk is unpredictable.

Generally speaking, the "black swan" event refers to an event that meets the following three characteristics:

1: unexpected.

2. It has great influence.

Although it is unexpected, human nature urges us to fabricate reasons for it afterwards, which is more or less explainable and predictable.

Black swan exists in all fields, no matter financial market, business, economy or personal life, it can't escape its control.