В статье в лингвокогнитивном аспекте исследуется функционирование идиом, включающих в свой состав члены лексико-семантической группы «Космос», в текстах русского языка XXI в. Выявляются лингвистические, когнитивные и культурологические факторы, влияющие на активность в употреблении членов фразеосемантической группы в XXI в., основные виды модификаций указанных идиом в современной речи, а также степень актуальности различных компонентов (наивных и научных, архаических и современных) той картины мира, которая репрезентируется с помощью исследуемых идиом. The article examines the functioning of 66 members of the phraseosemantic group “Cosmos” in the Russian language in the 21st century in cognitive and linguistic aspects. This group includes idioms that are members of the lexico-semantic group “Cosmos”. The material for the analysis was lexicographic sources and all contexts with these phraseological units recorded in the texts of the 2000–2020 Russian National Corpus (more than 3,000 occurrences). The methodological basis of the research was the idea of phraseology as a complex (linguistic, cognitive and cultural) phenomenon that plays an important role in the representation of the language picture of the world. The aim of the article was to identify the main trends in the functioning of idioms from one phraseosemantic group in the Russian language of the 21st century. The research used a complex method of system-semantic and cognitive-cultural analysis of language phenomena. According to lexicographic sources, the semantic, functional- stylistic and conceptual characteristics of the idioms, as well as their constituent lexemes from the “Cosmos” group are given. The relative statistics of the use of these idioms in modern speech, as well as the main factors affecting this process, are established. Specific (pragmatic, significative, formal-structural) features in the functioning of the phraseosemantic group “Cosmos” are revealed. Occasional and regular types of modification (transformation) of idioms in speech are specified, as well as their connection with the representation of the world picture, which is reflected in the internal form of phraseological units. It is noted that this picture includes components of naive and scientific, archaic and modern, primordial and borrowed ideas about extraterrestrial space, but is focused on reflecting mainly everyday, largely archaic views on the cosmic order. It is established that most members of the group function stably in modern texts, which determines the relevance of the transmitted picture of the world. The conceptual images that make up the basis of idiomatization are deliberate, which is indicated by the language game related to the actualization of the idioms’ inner form with formal and semantic modification (transformation) in the texts of the 21st century. The occasional speech errors in the use of the idioms have a cognitive basis and are associated with the neglect of the internal forms of these idioms. The presence of regular formal, semantic, functional and stylistic transformations indicates the relevance of the studied idioms for modern native speakers, as well as the main linguistic and cognitive trends in the development of the phraseological subsystem.