Excel, 10 bytes, score (145!)-(144!)(145!)-(144!) 0 Cracked!
I'm interested to see how this can be cracked.
=FACT(145)
Outputs 8.04792605747199E+251 (actually prints every digit when in "Number" format and a very wide column/merged columns, the most I could get it to print before they became ##### or #NUM!
=FACT(144)
Outputs 5.55029383273931E+249 (again, actually prints every digit [although it does turn most to 0's])
But... if you think it's okay if I can use un-printable numbers (likely not)
Excel, 10 bytes, score (999999999!)-(100000000999999998!)
=FACT(999999999)
Output is approximately 1e8448735636
=FACT(100000000999999998)
Output is approximately 1e768066876(1e8448735636)/999999999