We don't so much have a bubble, we have a manufactured market. High frequency trading algorithms are near 70% of the volume on any given trading day in every major market (nasdaq, NYSE, etc.).
The money to fund these algorithmic trading accounts is coming directly from central banks (The Fed and ECB) via "asset" purchases and swaps, this is where the insolvent banks take their worthless mortgage backed securities and put them up for collateral at a central bank who then lends them real money (via currency debasement) at near zero interest rates for their worthless paper, it's known that the federal reserve bank has lent out over 16 trillion dollars to foreign and domestic banks since the meltdown/heist of 2008. That money has to go somewhere to chase a yield so it goes into the "markets". With all that free money being thrown at stocks you get crazy stuff like Apple having a market cap of near a trillion dollars, this differs in my opinion from a bubble, this is levitation, as long as free money is being pumped into degenerate gambling/banking concerns the levitation will continue. So, really folks welcome to the centrally planned economy on steroids. It should be fun while it lasts.
And remember every time you hear that they are "pumping money into the economy" to check your bank accounts, they don't mean YOU, what they mean is that they are pumping more money into insolvent, degenerate banks so they can go play in the casino called "the markets".
If you have further interest, poke around for info on "high frequency trading".