Since January the Dow most of the time simply drift higher. Specially 3o mins before the close is -0.2 or -0.3% and then drifts to positive.
Why?
Simply since 9 January there is not much volume and mostly ( 55% ) is high frequency trades.
In this situation the American mum and dad keeps on buying US shares Exchange Traded Funds ETF.
These ETF have to allocate these new money and they do so in the last half an hour lifting the index.
Mistery solved.