Complex sentence: It is better represented on the territory of today Northern Bulgaria and was an early "Slav-Bulgarian" culture and its spread north of the Danube was caused probably by the temporary extension of the First Bulgarian Kingdom over some north-Danubian areas.
Split into simpler sentences: It is better represented on the territory of today Northern Bulgaria and was an early "Slav-Bulgarian" culture. Its spread north of the Danube was caused probably by the temporary extension of the First Bulgarian Kingdom over some north-Danubian areas.