These graphs tell the whole story of the current US stocks, bonds and real estate bubble:
Federal Reserve Balance Sheet, 2003 - 2015:
Source: Econbrowser.com
Federal Reserve Balance Sheet vs. S&P 500:
Source: Raymond James
Explanation: The US Fed buys less risky assets such as treasuries from market actors in order to pump liquidity into the system. The liquidity is invested by banks and others in stocks, bonds and real estate (the second chart illustrates the tight correlation between FED money printing and the value of the S&P 500). All the buying will push up asset prices. The FED recently stopped propping up the markets with liquidity (cash), making the bubble system super fragile. The fragility in combination with any external or internal shock (such as the Brexit) will eventually and most likely very soon popp the US bubble. As previously mentioned, the bubble has already popped in many other markets.
Disclosure: I wrote this article myself, and it expresses my own opinions. I am not receiving any compensation for it.