It is just created by the flue from the downstairs chimney as it sweeps across and comes up the side of the upstairs chimney. On mine I dropped a newspaper down the void and then sealed the space with steel, concrete and tiles. (I can see what looks like a bar code down the bottom of yours so maybe someone has done something simular)
In theory it should be solid brickwork below, but bricklayers often took shortcuts on the mass of brickwork in a chimney with loose rubble etc which settled.