The crisis revolved around the big investment banks. It arguably began with the default of hedge funds that invested in the secondary US mortgage market. French Bank BNP Paribas's step to suspend three similar funds exposed the US mortgage market crisis. The final nail in the coffin was when Lehman Brothers declared themselves bankrupt in September 2008.
Following which other major banks and corporations started failing. The interconnectivity of the world's financial system hugely affected banks globally. As a result, some banks restructured themselves while many closed down. After ten years, analysts say the financial crisis of 2007-09 highlighted the risk of concentration. But, the trend of larger banks and concentration of wealth is still intact.