Science: What is a Black hole?

What is a Black hole?

Mode	Cache item	Static	H	M	Primary store	H	M	S	I/O
Req	adhoc/filter_activitynames				default_request	6	1	3	-
App	core/capabilities	static accel.	178	1	default_application	1	0	0	180.9 KB
App	core/completion	static accel.	0	1	default_application	1	0	1	3.4 KB
Ses	core/coursecat				default_session	0	3	4	-
App	core/coursecattree	static accel.	2	2	default_application	2	0	0	7.3 KB
App	core/coursemodinfo				default_application	3	0	0	37.9 KB
App	core/databasemeta	static accel.	1	4	default_application	4	0	0	19.1 KB
App	core/eventinvalidation	static accel.	0	1	default_application	1	0	0	0.1 KB
App	core/htmlpurifier				default_application	1	0	0	0.0 KB
App	core/langmenu	static accel.	1	1	default_application	1	0	0	0.6 KB
Ses	core/navigation_expandcourse				default_session	0	1	2	-
App	core/observers	static accel.	0	2	default_application	2	0	0	21.1 KB
App	core/plugin_functions	static accel.	0	14	default_application	14	0	0	3.3 KB
App	core/plugin_manager				default_application	2	1	0	24.6 KB
App	core/roledefs	static accel.	0	1	default_application	1	0	0	1.5 KB
App	core/string	static accel.	145	36	default_application	36	0	0	463.0 KB
App	core/yuimodules				default_application	3	0	0	16.7 KB
App	tool_monitor/eventsubscriptions	static accel.	1	2	default_application	3	0	0	0.0 KB

Store name	Cache store	H	M	S	I/O
static accel.		328	65	0	-
default_application	cachestore_file	75	1	1	779.6 KB
default_request	cachestore_static	6	1	3	-
default_session	cachestore_session	0	4	6	-
Total		409	71	10	779.6 KB