They were trapped for seven days. God's work was able to be done in freedom: the dividing line between earth and sky, earth and ocean, the fecund fields with animals and birds, the oceans teeming with fish and monsters, the two legged animals - human beings - created to carry God's hope.
But the forces of chaos, of tohu and bohu, were chained for those seven days; trapped and kept away from the great work of creation.
There was order at work: chaos was trapped. There was fertility abounding: destruction was stayed. There was ingenuity in creation: blankness was put down.
Six days God created, and on the seventh God rested. Some Jews believe that we are still in the seventh day. Some Christians believe that the day of rest is yet to come. While creation continues, the seventh day continues too. And the tohu and bohu, mischievous and evil remain trapped.
But the thing is, trapped they are powerless. Trapped for ever, they cannot rise again.
The tohu and bohu are trapped and dead!
The forces of creation: the ingenuity, the creativity, the fertility, the order can rejoice and continue to rejoice. They are free for ever.
That is why we say "Alleluia!"