I know no logical reason why a gap should get filled. (Many people say that gaps get filled, but I have seen too many unfilled gaps and hence don't believe any such theory).
Some gaps get filled and some never get filled.
If all gaps would get filled, why would there be gaps in the first place?
Suppose stock makes a low of 44 on one day.
Next day the stock opens at 41.
If all gaps are going to be filled, why would anyone sell at 41 and if the stock is certainly going to touch 44 in next few days, everyone should be buying the stock at 41...isn't it?
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Gaps can act as a crucial support/resistance level.
Gaps leave one side of the market in a shock.
In the above example, bulls would have been left in a shock when the stock opened at 41.
The usual thinking of such a shocked side is
"if the price comes back to the previous day's low/close, I will liquidate my position".
This means if the stock were to go up to 44 in next few days, the longs (bulls) would prefer to close their positions.
Thus, this level (44) would now become a strong resistance as many longs (bulls) would like to close their positions at this level.
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There is a thread on
Dow Jones Industrial Average in the
World Markets forum.